Living Standard — Last Updated 3 October 2023
innerText and outerText propertiesbody elementarticle elementsection elementnav elementaside elementh1, h2, h3, h4, h5, and h6
elementshgroup elementheader elementfooter elementaddress elementp elementhr elementpre elementblockquote elementol elementul elementmenu elementli elementdl elementdt elementdd elementfigure elementfigcaption elementmain elementsearch elementdiv elementa elementem elementstrong elementsmall elements elementcite elementq elementdfn elementabbr elementruby elementrt elementrp elementdata elementtime elementcode elementvar elementsamp elementkbd elementsub and sup elementsi elementb elementu elementmark elementbdi elementbdo elementspan elementbr elementwbr elementa and area elementsa and area elementsalternate"author"bookmark"canonical"dns-prefetch"external"help"icon"license"manifest"modulepreload"nofollow"noopener"noreferrer"opener"pingback"preconnect"prefetch"preload"search"stylesheet"tag"picture elementsource elementimg elementsource,
img, and link elementsiframe elementembed elementobject elementvideo elementaudio elementtrack elementTrackEvent interfacemap elementarea elementtable elementcaption elementcolgroup elementcol elementtbody elementthead elementtfoot elementtr elementtd elementth elementtd and th elementsform elementlabel elementinput elementtype attributetype=hidden)type=text) state and Search state (type=search)type=tel)type=url)type=email)type=password)type=date)type=month)type=week)type=time)type=datetime-local)type=number)type=range)type=color)type=checkbox)type=radio)type=file)type=submit)type=image)type=reset)type=button)input element attributesmaxlength and minlength attributessize attributereadonly attributerequired attributemultiple attributepattern attributemin and max attributesstep attributelist attributeplaceholder attributeinput element APIsbutton elementselect elementdatalist elementoptgroup elementoption elementtextarea elementoutput elementprogress elementmeter elementfieldset elementlegend elementname attributedirname attributemaxlength attributeminlength attributedisabled attributeSubmitEvent interfaceFormDataEvent interfacedetails elementsummary elementa element to define a commandbutton element to define a commandinput element to define a commandoption element to define a commandaccesskey attribute
on a legend element to define a commandaccesskey
attribute to define a command on other elementsdialog elementscript elementnoscript elementtemplate elementslot elementcanvas elementPath2D objectsImageBitmap rendering contextOffscreenCanvas interfacecanvas elementsCustomElementRegistry interfacehidden attributecontenteditable content attributedesignMode getter and setterinputmode attributeenterkeyhint
attributepopover attributeWindow,
WindowProxy, and Location objectsWindow objectWindowProxy exotic objectLocation interfaceHistory interfacemultipart/x-mixed-replace
documentsX-Frame-Options` headerRefresh` headerWindowOrWorkerGlobalScope mixinbutton elementdetails and summary elementsinput element as a text entry widgetinput element as domain-specific widgetsinput element as a range controlinput element as a color
wellinput element as a checkbox and radio button widgetsinput element as a file upload controlinput element as a buttonmarquee elementmeter elementprogress elementselect elementtextarea elementThis specification defines a big part of the web platform, in lots of detail. Its place in the web platform specification stack relative to other specifications can be best summed up as follows:
This section is non-normative.
In short: Yes.
In more length: the term "HTML5" is widely used as a buzzword to refer to modern web technologies, many of which (though by no means all) are developed at the WHATWG. This document is one such; others are available from .
This section is non-normative.
HTML is the World Wide Web's core markup language. Originally, HTML was primarily designed as a language for semantically describing scientific documents. Its general design, however, has enabled it to be adapted, over the subsequent years, to describe a number of other types of documents and even applications.
This section is non-normative.
This specification is intended for authors of documents and scripts that use the features defined in this specification, implementers of tools that operate on pages that use the features defined in this specification, and individuals wishing to establish the correctness of documents or implementations with respect to the requirements of this specification.
This document is probably not suited to readers who do not already have at least a passing familiarity with web technologies, as in places it sacrifices clarity for precision, and brevity for completeness. More approachable tutorials and authoring guides can provide a gentler introduction to the topic.
In particular, familiarity with the basics of DOM is necessary for a complete understanding of some of the more technical parts of this specification. An understanding of Web IDL, HTTP, XML, Unicode, character encodings, JavaScript, and CSS will also be helpful in places but is not essential.
This section is non-normative.
This specification is limited to providing a semantic-level markup language and associated semantic-level scripting APIs for authoring accessible pages on the web ranging from static documents to dynamic applications.
The scope of this specification does not include providing mechanisms for media-specific customization of presentation (although default rendering rules for web browsers are included at the end of this specification, and several mechanisms for hooking into CSS are provided as part of the language).
The scope of this specification is not to describe an entire operating system. In particular, hardware configuration software, image manipulation tools, and applications that users would be expected to use with high-end workstations on a daily basis are out of scope. In terms of applications, this specification is targeted specifically at applications that would be expected to be used by users on an occasional basis, or regularly but from disparate locations, with low CPU requirements. Examples of such applications include online purchasing systems, searching systems, games (especially multiplayer online games), public telephone books or address books, communications software (email clients, instant messaging clients, discussion software), document editing software, etc.
This section is non-normative.
For its first five years (1990-1995), HTML went through a number of revisions and experienced a number of extensions, primarily hosted first at CERN, and then at the IETF.
With the creation of the W3C, HTML's development changed venue again. A first abortive attempt at extending HTML in 1995 known as HTML 3.0 then made way to a more pragmatic approach known as HTML 3.2, which was completed in 1997. HTML4 quickly followed later that same year.
The following year, the W3C membership decided to stop evolving HTML and instead begin work on an XML-based equivalent, called XHTML. This effort started with a reformulation of HTML4 in XML, known as XHTML 1.0, which added no new features except the new serialization, and which was completed in 2000. After XHTML 1.0, the W3C's focus turned to making it easier for other working groups to extend XHTML, under the banner of XHTML Modularization. In parallel with this, the W3C also worked on a new language that was not compatible with the earlier HTML and XHTML languages, calling it XHTML2.
Around the time that HTML's evolution was stopped in 1998, parts of the API for HTML developed by browser vendors were specified and published under the name DOM Level 1 (in 1998) and DOM Level 2 Core and DOM Level 2 HTML (starting in 2000 and culminating in 2003). These efforts then petered out, with some DOM Level 3 specifications published in 2004 but the working group being closed before all the Level 3 drafts were completed.
In 2003, the publication of XForms, a technology which was positioned as the next generation of web forms, sparked a renewed interest in evolving HTML itself, rather than finding replacements for it. This interest was borne from the realization that XML's deployment as a web technology was limited to entirely new technologies (like RSS and later Atom), rather than as a replacement for existing deployed technologies (like HTML).
A proof of concept to show that it was possible to extend HTML4's forms to provide many of the features that XForms 1.0 introduced, without requiring browsers to implement rendering engines that were incompatible with existing HTML web pages, was the first result of this renewed interest. At this early stage, while the draft was already publicly available, and input was already being solicited from all sources, the specification was only under Opera Software's copyright.
The idea that HTML's evolution should be reopened was tested at a W3C workshop in 2004, where some of the principles that underlie the HTML5 work (described below), as well as the aforementioned early draft proposal covering just forms-related features, were presented to the W3C jointly by Mozilla and Opera. The proposal was rejected on the grounds that the proposal conflicted with the previously chosen direction for the web's evolution; the W3C staff and membership voted to continue developing XML-based replacements instead.
Shortly thereafter, Apple, Mozilla, and Opera jointly announced their intent to continue working on the effort under the umbrella of a new venue called the WHATWG. A public mailing list was created, and the draft was moved to the WHATWG site. The copyright was subsequently amended to be jointly owned by all three vendors, and to allow reuse of the specification.
The WHATWG was based on several core principles, in particular that technologies need to be backwards compatible, that specifications and implementations need to match even if this means changing the specification rather than the implementations, and that specifications need to be detailed enough that implementations can achieve complete interoperability without reverse-engineering each other.
The latter requirement in particular required that the scope of the HTML5 specification include what had previously been specified in three separate documents: HTML4, XHTML1, and DOM2 HTML. It also meant including significantly more detail than had previously been considered the norm.
In 2006, the W3C indicated an interest to participate in the development of HTML5 after all, and in 2007 formed a working group chartered to work with the WHATWG on the development of the HTML5 specification. Apple, Mozilla, and Opera allowed the W3C to publish the specification under the W3C copyright, while keeping a version with the less restrictive license on the WHATWG site.
For a number of years, both groups then worked together. In 2011, however, the groups came to the conclusion that they had different goals: the W3C wanted to publish a "finished" version of "HTML5", while the WHATWG wanted to continue working on a Living Standard for HTML, continuously maintaining the specification rather than freezing it in a state with known problems, and adding new features as needed to evolve the platform.
In 2019, the WHATWG and W3C signed an agreement to collaborate on a single version of HTML going forward: this document.
This section is non-normative.
It must be admitted that many aspects of HTML appear at first glance to be nonsensical and inconsistent.
HTML, its supporting DOM APIs, as well as many of its supporting technologies, have been developed over a period of several decades by a wide array of people with different priorities who, in many cases, did not know of each other's existence.
Features have thus arisen from many sources, and have not always been designed in especially consistent ways. Furthermore, because of the unique characteristics of the web, implementation bugs have often become de-facto, and now de-jure, standards, as content is often unintentionally written in ways that rely on them before they can be fixed.
Despite all this, efforts have been made to adhere to certain design goals. These are described in the next few subsections.
This section is non-normative.
To avoid exposing web authors to the complexities of multithreading, the HTML and DOM APIs are designed such that no script can ever detect the simultaneous execution of other scripts. Even with workers, the intent is that the behavior of implementations can be thought of as completely serializing the execution of all scripts in all globals.
The exception to this general design principle is the JavaScript SharedArrayBuffer
class. Using SharedArrayBuffer objects, it can in fact be observed that scripts in
other agents are executing simultaneously. Furthermore, due to the
JavaScript memory model, there are situations which not only are un-representable via serialized
script execution, but also un-representable via serialized statement execution
among those scripts.
This section is non-normative.
This specification interacts with and relies on a wide variety of other specifications. In certain circumstances, unfortunately, conflicting needs have led to this specification violating the requirements of these other specifications. Whenever this has occurred, the transgressions have each been noted as a "willful violation", and the reason for the violation has been noted.
This section is non-normative.
HTML has a wide array of extensibility mechanisms that can be used for adding semantics in a safe manner:
Authors can use the class attribute to extend elements,
effectively creating their own elements, while using the most applicable existing "real" HTML
element, so that browsers and other tools that don't know of the extension can still support it
somewhat well. This is the tack used by microformats, for example.
Authors can include data for inline client-side scripts or server-side site-wide scripts
to process using the data-*="" attributes. These are guaranteed
to never be touched by browsers, and allow scripts to include data on HTML elements that scripts
can then look for and process.
Authors can use the <meta name="" content=""> mechanism to
include page-wide metadata.
Authors can use the rel="" mechanism to annotate
links with specific meanings by registering extensions to
the predefined set of link types. This is also used by microformats.
Authors can embed raw data using the <script type="">
mechanism with a custom type, for further handling by inline or server-side scripts.
Authors can extend APIs using the JavaScript prototyping mechanism. This is widely used by script libraries, for instance.
Authors can use the microdata feature (the itemscope="" and itemprop=""
attributes) to embed nested name-value pairs of data to be shared with other applications and
sites.
Authors can define, share, and use custom elements to extend the vocabulary of HTML. The requirements of valid custom element names ensure forward compatibility (since no elements will be added to HTML, SVG, or MathML with hyphen-containing local names in the future).
This section is non-normative.
This specification defines an abstract language for describing documents and applications, and some APIs for interacting with in-memory representations of resources that use this language.
The in-memory representation is known as "DOM HTML", or "the DOM" for short.
There are various concrete syntaxes that can be used to transmit resources that use this abstract language, two of which are defined in this specification.
The first such concrete syntax is the HTML syntax. This is the format suggested for most
authors. It is compatible with most legacy web browsers. If a document is transmitted with the
text/html MIME type, then it will be processed as an HTML document by
web browsers. This specification defines the latest HTML syntax, known simply as "HTML".
The second concrete syntax is XML. When a document is transmitted with an XML MIME
type, such as application/xhtml+xml, then it is treated as an XML document by
web browsers, to be parsed by an XML processor. Authors are reminded that the processing for XML
and HTML differs; in particular, even minor syntax errors will prevent a document labeled as XML
from being rendered fully, whereas they would be ignored in the HTML syntax.
The XML syntax for HTML was formerly referred to as "XHTML", but this specification does not use that term (among other reasons, because no such term is used for the HTML syntaxes of MathML and SVG).
The DOM, the HTML syntax, and the XML syntax cannot all represent the same content. For
example, namespaces cannot be represented using the HTML syntax, but they are supported in the DOM
and in the XML syntax. Similarly, documents that use the noscript feature can be
represented using the HTML syntax, but cannot be represented with the DOM or in the XML syntax.
Comments that contain the string "-->" can only be represented in the
DOM, not in the HTML and XML syntaxes.
This section is non-normative.
This specification is divided into the following major sections:
EventSource, and a two-way full-duplex socket protocol for scripts known as Web
Sockets.There are also some appendices, listing obsolete features and IANA considerations, and several indices.
This specification should be read like all other specifications. First, it should be read cover-to-cover, multiple times. Then, it should be read backwards at least once. Then it should be read by picking random sections from the contents list and following all the cross-references.
As described in the conformance requirements section below, this specification describes conformance criteria for a variety of conformance classes. In particular, there are conformance requirements that apply to producers, for example authors and the documents they create, and there are conformance requirements that apply to consumers, for example web browsers. They can be distinguished by what they are requiring: a requirement on a producer states what is allowed, while a requirement on a consumer states how software is to act.
For example, "the foo attribute's value must be a valid
integer" is a requirement on producers, as it lays out the allowed values; in contrast,
the requirement "the foo attribute's value must be parsed using the
rules for parsing integers" is a requirement on consumers, as it describes how to
process the content.
Requirements on producers have no bearing whatsoever on consumers.
Continuing the above example, a requirement stating that a particular attribute's value is constrained to being a valid integer emphatically does not imply anything about the requirements on consumers. It might be that the consumers are in fact required to treat the attribute as an opaque string, completely unaffected by whether the value conforms to the requirements or not. It might be (as in the previous example) that the consumers are required to parse the value using specific rules that define how invalid (non-numeric in this case) values are to be processed.
This is a definition, requirement, or explanation.
This is a note.
This is an example.
This is an open issue.
This is a warning.
[Exposed =Window ]
interface Example {
// this is an IDL definition
};variable = object.method([optionalArgument])This is a note to authors describing the usage of an interface.
/* this is a CSS fragment */ The defining instance of a term is marked up like this. Uses of that term are marked up like this or like this.
The defining instance of an element, attribute, or API is marked up like this. References to that element, attribute, or API are marked up
like this.
Other code fragments are marked up like this.
Variables are marked up like this.
In an algorithm, steps in synchronous sections are marked with ⌛.
In some cases, requirements are given in the form of lists with conditions and corresponding requirements. In such cases, the requirements that apply to a condition are always the first set of requirements that follow the condition, even in the case of there being multiple sets of conditions for those requirements. Such cases are presented as follows:
This section is non-normative.
A basic HTML document looks like this:
<!DOCTYPE html>
< html lang = "en" >
< head >
< title > Sample page</ title >
</ head >
< body >
< h1 > Sample page</ h1 >
< p > This is a < a href = "demo.html" > simple</ a > sample.</ p >
<!-- this is a comment -->
</ body >
</ html > HTML documents consist of a tree of elements and text. Each element is denoted in the source by
a start tag, such as "<body>", and
an end tag, such as "</body>".
(Certain start tags and end tags can in certain cases be omitted and are implied by other tags.)
Tags have to be nested such that elements are all completely within each other, without overlapping:
< p > This is < em > very < strong > wrong</ em > !</ strong ></ p > < p > This < em > is < strong > correct</ strong > .</ em ></ p > This specification defines a set of elements that can be used in HTML, along with rules about the ways in which the elements can be nested.
Elements can have attributes, which control how the elements work. In the example below, there
is a hyperlink, formed using the a element and its href attribute:
< a href = "demo.html" > simple</ a > Attributes are placed inside the start tag, and consist
of a name and a value, separated by an "=" character.
The attribute value can remain unquoted if it doesn't contain ASCII
whitespace or any of " ' ` = < or >. Otherwise, it has to be quoted using either single or double quotes. The
value, along with the "=" character, can be omitted altogether if the value
is the empty string.
<!-- empty attributes -->
< input name = address disabled >
< input name = address disabled = "" >
<!-- attributes with a value -->
< input name = address maxlength = 200 >
< input name = address maxlength = '200' >
< input name = address maxlength = "200" > HTML user agents (e.g., web browsers) then parse this markup, turning it into a DOM (Document Object Model) tree. A DOM tree is an in-memory representation of a document.
DOM trees contain several kinds of nodes, in particular a The markup snippet at the top of this section would be
turned into the following DOM tree: The document element of this tree is the There are many more The This DOM tree can be manipulated from scripts in the page. Scripts (typically in JavaScript)
are small programs that can be embedded using the Each element in the DOM tree is represented by an object, and these objects have APIs so that
they can be manipulated. For instance, a link (e.g. the Since DOM trees are used as the way to represent HTML documents when they are processed and
presented by implementations (especially interactive implementations like web browsers), this
specification is mostly phrased in terms of DOM trees, instead of the markup described above. HTML documents represent a media-independent description of interactive content. HTML documents
might be rendered to a screen, or through a speech synthesizer, or on a braille display. To
influence exactly how such rendering takes place, authors can use a styling language such as
CSS. In the following example, the page has been made yellow-on-blue using CSS. For more details on how to use HTML, authors are encouraged to consult tutorials and guides.
Some of the examples included in this specification might also be of use, but the novice author is
cautioned that this specification, by necessity, defines the language with a level of detail that
might be difficult to understand at first. This section is non-normative. When HTML is used to create interactive sites, care needs to be taken to avoid introducing
vulnerabilities through which attackers can compromise the integrity of the site itself or of the
site's users. A comprehensive study of this matter is beyond the scope of this document, and authors are
strongly encouraged to study the matter in more detail. However, this section attempts to provide
a quick introduction to some common pitfalls in HTML application development. The security model of the web is based on the concept of "origins", and correspondingly many of
the potential attacks on the web involve cross-origin actions. [ORIGIN] When accepting untrusted input, e.g. user-generated content such as text comments, values in
URL parameters, messages from third-party sites, etc, it is imperative that the data be
validated before use, and properly escaped when displayed. Failing to do this can allow a
hostile user to perform a variety of attacks, ranging from the potentially benign, such as
providing bogus user information like a negative age, to the serious, such as running scripts
every time a user looks at a page that includes the information, potentially propagating the
attack in the process, to the catastrophic, such as deleting all data in the server. When writing filters to validate user input, it is imperative that filters always be
safelist-based, allowing known-safe constructs and disallowing all other input. Blocklist-based
filters that disallow known-bad inputs and allow everything else are not secure, as not
everything that is bad is yet known (for example, because it might be invented in the
future). For example, suppose a page looked at its URL's query string to determine what to display,
and the site then redirected the user to that page to display a message, as in: If the message was just displayed to the user without escaping, a hostile attacker could
then craft a URL that contained a script element: If the attacker then convinced a victim user to visit this page, a script of the attacker's
choosing would run on the page. Such a script could do any number of hostile actions, limited
only by what the site offers: if the site is an e-commerce shop, for instance, such a script
could cause the user to unknowingly make arbitrarily many unwanted purchases. This is called a cross-site scripting attack. There are many constructs that can be used to try to trick a site into executing code. Here
are some that authors are encouraged to consider when writing safelist filters: If a site allows a user to make form submissions with user-specific side-effects, for example
posting messages on a forum under the user's name, making purchases, or applying for a passport,
it is important to verify that the request was made by the user intentionally, rather than by
another site tricking the user into making the request unknowingly. This problem exists because HTML forms can be submitted to other origins. Sites can prevent such attacks by populating forms with user-specific hidden tokens, or by
checking ` A page that provides users with an interface to perform actions that the user might not wish
to perform needs to be designed so as to avoid the possibility that users can be tricked into
activating the interface. One way that a user could be so tricked is if a hostile site places the victim site in a
small To avoid this, sites that do not expect to be used in frames are encouraged to only enable
their interface if they detect that they are not in a frame (e.g. by comparing the This section is non-normative. Scripts in HTML have "run-to-completion" semantics, meaning that the browser will generally run
the script uninterrupted before doing anything else, such as firing further events or continuing
to parse the document. On the other hand, parsing of HTML files happens incrementally, meaning that
the parser can pause at any point to let scripts run. This is generally a good thing, but it does
mean that authors need to be careful to avoid hooking event handlers after the events could have
possibly fired. There are two techniques for doing this reliably: use event handler content
attributes, or create the element and add the event handlers in the same script. The latter
is safe because, as mentioned earlier, scripts are run to completion before further events can
fire. One way this could manifest itself is with Here, the author uses the If the element is being added by script, then so long as the event handlers are added in the
same script, the event will still not be missed: However, if the author first created the This section is non-normative. Authors are encouraged to make use of conformance checkers (also known as validators) to
catch common mistakes. The WHATWG maintains a list of such tools at: 1.11 Conformance requirements for authors
This section is non-normative. Unlike previous versions of the HTML specification, this specification defines in some detail
the required processing for invalid documents as well as valid documents. However, even though the processing of invalid content is in most cases well-defined,
conformance requirements for documents are still important: in practice, interoperability (the
situation in which all implementations process particular content in a reliable and identical or
equivalent way) is not the only goal of document conformance requirements. This section details
some of the more common reasons for still distinguishing between a conforming document and one
with errors. This section is non-normative. The majority of presentational features from previous versions of HTML are no longer allowed.
Presentational markup in general has been found to have a number of problems: While it is possible to use presentational markup in a way that provides users of assistive
technologies (ATs) with an acceptable experience (e.g. using ARIA), doing so is significantly
more difficult than doing so when using semantically-appropriate markup. Furthermore, even using
such techniques doesn't help make pages accessible for non-AT non-graphical users, such as users
of text-mode browsers. Using media-independent markup, on the other hand, provides an easy way for documents to be
authored in such a way that they work for more users (e.g. users of text browsers). It is significantly easier to maintain a site written in such a way that the markup is
style-independent. For example, changing the color of a site that uses Presentational markup tends to be much more redundant, and thus results in larger document
sizes. For those reasons, presentational markup has been removed from HTML in this version. This
change should not come as a surprise; HTML4 deprecated presentational markup many years ago and
provided a mode (HTML4 Transitional) to help authors move away from presentational markup; later,
XHTML 1.1 went further and obsoleted those features altogether. The only remaining presentational markup features in HTML are the It is also worth noting that some elements that were previously presentational have been
redefined in this specification to be media-independent: This section is non-normative. The syntax of HTML is constrained to avoid a wide variety of problems. Certain invalid syntax constructs, when parsed, result in DOM trees that are highly
unintuitive. To allow user agents to be used in controlled environments without having to implement the
more bizarre and convoluted error handling rules, user agents are permitted to fail whenever
encountering a parse error. Some error-handling behavior, such as the behavior for the When a user agent based on XML is connected to an HTML parser, it is possible that certain
invariants that XML enforces, such as element or attribute names never contain multiple colons,
will be violated by an HTML file. Handling this can require that the parser coerce the HTML DOM
into an XML-compatible infoset. Most syntax constructs that require such handling are considered
invalid. (Comments containing two consecutive hyphens, or ending with a hyphen, are exceptions
that are allowed in the HTML syntax.) Certain syntax constructs can result in disproportionately poor performance. To discourage the
use of such constructs, they are typically made non-conforming. For example, the following markup results in poor performance, since all the unclosed
The resulting DOM for this fragment would be: There are syntax constructs that, for historical reasons, are relatively fragile. To help
reduce the number of users who accidentally run into such problems, they are made
non-conforming. For example, the parsing of certain named character references in attributes happens even
with the closing semicolon being omitted. It is safe to include an ampersand followed by
letters that do not form a named character reference, but if the letters are changed to a
string that does form a named character reference, they will be interpreted as that
character instead. In this fragment, the attribute's value is " In the following fragment, however, the attribute's value is actually " To avoid this problem, all named character references are required to end with a semicolon,
and uses of named character references without a semicolon are flagged as errors. Thus, the correct way to express the above cases is as
follows: Certain syntax constructs are known to cause especially subtle or serious problems in legacy
user agents, and are therefore marked as non-conforming to help authors avoid them. For example, this is why the U+0060 GRAVE ACCENT character (`) is not allowed in unquoted
attributes. In certain legacy user agents, it is sometimes treated as a
quote character. Another example of this is the DOCTYPE, which is required to trigger no-quirks
mode, because the behavior of legacy user agents in quirks mode is often
largely undocumented. Certain restrictions exist purely to avoid known security problems. For example, the restriction on using UTF-7 exists purely to avoid authors falling prey to a
known cross-site-scripting attack using UTF-7. [UTF7] Markup where the author's intent is very unclear is often made non-conforming. Correcting
these errors early makes later maintenance easier. When a user makes a simple typo, it is helpful if the error can be caught early, as this can
save the author a lot of debugging time. This specification therefore usually considers it an
error to use element names, attribute names, and so forth, that do not match the names defined
in this specification. For example, if the author typed In order to allow the language syntax to be extended in the future, certain otherwise
harmless features are disallowed. For example, "attributes" in end tags are ignored currently, but they are invalid, in case a
future change to the language makes use of that syntax feature without conflicting with
already-deployed (and valid!) content. Some authors find it helpful to be in the practice of always quoting all attributes and always
including all optional tags, preferring the consistency derived from such custom over the minor
benefits of terseness afforded by making use of the flexibility of the HTML syntax. To aid such
authors, conformance checkers can provide modes of operation wherein such conventions are
enforced. This section is non-normative. Beyond the syntax of the language, this specification also places restrictions on how elements
and attributes can be specified. These restrictions are present for similar reasons: To avoid misuse of elements with defined meanings, content models are defined that restrict
how elements can be nested when such nestings would be of dubious value. For example, this specification disallows nesting a Similarly, to draw the author's attention to mistakes in the use of elements, clear
contradictions in the semantics expressed are also considered conformance errors. In the fragments below, for example, the semantics are nonsensical: a separator cannot
simultaneously be a cell, nor can a radio button be a progress bar. Another example is the restrictions on the content models of the
Certain elements have default styles or behaviors that make certain combinations likely to
lead to confusion. Where these have equivalent alternatives without this problem, the confusing
combinations are disallowed. For example, Another example would be the way interactive content cannot be
nested. For example, a Sometimes, something is disallowed because allowing it would likely cause author
confusion. For example, setting the Some conformance errors simplify the language that authors need to learn. For example, the Certain elements are parsed in somewhat eccentric ways (typically for historical reasons),
and their content model restrictions are intended to avoid exposing the author to these
issues. For example, a It is parsed exactly like the following: Some errors are intended to help prevent script problems that would be hard to debug. This is why, for instance, it is non-conforming to have two Some constructs are disallowed because historically they have been the cause of a lot of
wasted authoring time, and by encouraging authors to avoid making them, authors can save time in
future efforts. For example, a Some authors like to write files that can be interpreted as both XML and HTML with similar
results. Though this practice is discouraged in general due to the myriad of subtle
complications involved (especially when involving scripting, styling, or any kind of automated
serialization), this specification has a few restrictions intended to at least somewhat mitigate
the difficulties. This makes it easier for authors to use this as a transitionary step when
migrating between the HTML and XML syntaxes. For example, there are somewhat complicated rules surrounding the Another example would be the restrictions on the values of As with the restrictions on the syntax intended to allow for new syntax in future revisions
of the language, some restrictions on the content models of elements and values of attributes
are intended to allow for future expansion of the HTML vocabulary. For example, limiting the values of the Certain restrictions are intended to support the restrictions made by other
specifications. For example, requiring that attributes that take media query lists use only
valid media query lists reinforces the importance of following the conformance rules of
that specification. This section is non-normative. The following documents might be of interest to readers of this specification. This Architectural Specification provides authors of specifications, software
developers, and content developers with a common reference for interoperable text manipulation on
the World Wide Web, building on the Universal Character Set, defined jointly by the Unicode
Standard and ISO/IEC 10646. Topics addressed include use of the terms 'character', 'encoding' and
'string', a reference processing model, choice and identification of character encodings,
character escaping, and string indexing. Because Unicode contains such a large number of characters and incorporates
the varied writing systems of the world, incorrect usage can expose programs or systems to
possible security attacks. This is especially important as more and more products are
internationalized. This document describes some of the security considerations that programmers,
system analysts, standards developers, and users should take into account, and provides specific
recommendations to reduce the risk of problems. Web Content Accessibility Guidelines (WCAG) covers a wide range of
recommendations for making web content more accessible. Following these guidelines will make
content accessible to a wider range of people with disabilities, including blindness and low
vision, deafness and hearing loss, learning disabilities, cognitive limitations, limited
movement, speech disabilities, photosensitivity and combinations of these. Following these
guidelines will also often make your web content more usable to users in
general. This specification provides guidelines for designing web content
authoring tools that are more accessible for people with disabilities. An authoring tool that
conforms to these guidelines will promote accessibility by providing an accessible user interface
to authors with disabilities as well as by enabling, supporting, and promoting the production of
accessible web content by all authors. This document provides guidelines for designing user agents that
lower barriers to web accessibility for people with disabilities. User agents include browsers
and other types of software that retrieve and render web content. A user agent that conforms to
these guidelines will promote accessibility through its own user interface and through other
internal facilities, including its ability to communicate with other technologies (especially
assistive technologies). Furthermore, all users, not just users with disabilities, should find
conforming user agents to be more usable. This specification depends on Infra. [INFRA] This specification refers to both HTML and XML attributes and IDL attributes, often in the same
context. When it is not clear which is being referred to, they are referred to as content attributes for HTML and XML attributes,
and IDL attributes for those defined on IDL interfaces. Similarly, the term
"properties" is used for both JavaScript object properties and CSS properties. When these are
ambiguous they are qualified as object properties and CSS properties respectively. Generally, when the specification states that a feature applies to the HTML syntax
or the XML syntax, it also includes the other. When a feature specifically only
applies to one of the two languages, it is called out by explicitly stating that it does not apply
to the other format, as in "for HTML, ... (this does not apply to XML)". This specification uses the term document to refer to any use of HTML,
ranging from short static documents to long essays or reports with rich multimedia, as well as to
fully-fledged interactive applications. The term is used to refer both to In the context of the DOM structures, the terms HTML
document and XML document are used as defined in
DOM, and refer specifically to two different modes that In the context of byte streams, the term HTML document refers to resources labeled as
For simplicity, terms such as shown, displayed, and
visible might sometimes be used when referring to the way a document is
rendered to the user. These terms are not meant to imply a visual medium; they must be considered
to apply to other media in equivalent ways. To run steps in parallel means those steps are to be run, one after another,
at the same time as other logic in the standard (e.g., at the same time as the event
loop). This standard does not define the precise mechanism by which this is achieved, be it
time-sharing cooperative multitasking, fibers, threads, processes, using different hyperthreads,
cores, CPUs, machines, etc. By contrast, an operation that is to run immediately must
interrupt the currently running task, run itself, and then resume the previously running task. For guidance on writing specifications that leverage parallelism, see Dealing with the event loop from other specifications. To avoid race conditions between different in parallel algorithms that operate on
the same data, a parallel queue can be used. A parallel queue represents a queue of algorithm steps that must be run in
series. A parallel queue has an algorithm queue (a queue),
initially empty. To enqueue steps to a parallel queue,
enqueue the algorithm steps to the parallel queue's algorithm
queue. To start a new parallel queue, run the following steps: Let parallelQueue be a new parallel queue. Run the following steps in parallel: While true: Let steps be the result of dequeueing from
parallelQueue's algorithm queue. If steps is not nothing, then run steps. Assert: running steps did not throw an exception, as steps
running in parallel are not allowed to throw. Implementations are not expected to implement this as a continuously running
loop. Algorithms in standards are to be easy to understand and are not necessarily great for
battery life or performance. Return parallelQueue. Steps running in parallel can themselves run other steps in in
parallel. E.g., inside a parallel queue it can be useful to run a series of
steps in parallel with the queue. Imagine a standard defined nameList (a list), along with a method to
add a name to nameList, unless nameList already contains name, in which case it rejects. The following solution suffers from race conditions: Let p be a new promise. Run the following steps in parallel: Return p. Two invocations of the above could run simultaneously, meaning name isn't in
nameList during step 2.1, but it might be added before step 2.3 runs,
meaning name ends up in nameList twice. Parallel queues solve this. The standard would let nameListQueue be the result of
starting a new parallel queue, then: Let p be a new promise. Enqueue the following steps to nameListQueue: Return p. The steps would now queue and the race is avoided. The specification uses the term supported when referring to whether a user
agent has an implementation capable of decoding the semantics of an external resource. A format or
type is said to be supported if the implementation can process an external resource of that
format or type without critical aspects of the resource being ignored. Whether a specific resource
is supported can depend on what features of the resource's format are in use. For example, a PNG image would be considered to be in a supported format if its
pixel data could be decoded and rendered, even if, unbeknownst to the implementation, the image
also contained animation data. An MPEG-4 video file would not be considered to be in a supported format if the
compression format used was not supported, even if the implementation could determine the
dimensions of the movie from the file's metadata. What some specifications, in particular the HTTP specifications, refer to as a
representation is referred to in this specification as a resource.
[HTTP] A resource's critical subresources are those that the resource needs to have
available to be correctly processed. Which resources are considered critical or not is defined by
the specification that defines the resource's format. For CSS style sheets, we tentatively define here that
their critical subresources are other style sheets imported via This definition is not fully interoperable; furthermore, some user agents seem to
count resources like background images or web fonts as critical subresources. Ideally, the CSS
Working Group would define this; see w3c/csswg-drafts issue #1088 to track
progress on that front. To ease migration from HTML to XML, user agents conforming to this
specification will place elements in HTML in the Except where otherwise stated, all elements defined or mentioned in this specification are in
the HTML namespace (" The term element type is used to refer to the set of elements that have a given
local name and namespace. For example, Attribute names are said to be XML-compatible if they match the When it is stated that some element or attribute is ignored, or
treated as some other value, or handled as if it was something else, this refers only to the
processing of the node after it is in the DOM. A user agent must not mutate the
DOM in such situations. A content attribute is said to change value only if its new value is
different than its previous value; setting an attribute to a value it already has does not change
it. The term empty, when used for an attribute value, An HTML element can have specific HTML element insertion steps defined for the
element's local name. Similarly, an HTML element
can have specific HTML element removing steps defined for the element's local name. The insertion steps for the HTML Standard, given
insertedNode, are defined as the following: If insertedNode is an element whose namespace is the HTML namespace, and this
standard defines HTML element insertion steps
for insertedNode's local name, then
run the corresponding HTML element insertion steps given
insertedNode. If insertedNode is a form-associated element or the ancestor of a
form-associated element, then: If the form-associated element's parser inserted flag is set,
then return. The removing steps for the HTML Standard, given
removedNode and oldParent, are defined as the following: Let document be removedNode's node document. If document's focused area is
removedNode, then set document's focused area to document's viewport, and set
document's relevant global object's navigation API's focus changed during ongoing
navigation to false. This does not perform the unfocusing steps,
focusing steps, or focus update steps, and thus no If removedNode is an element whose namespace is the HTML namespace, and this
standard defines HTML element removing steps
for removedNode's local name, then
run the corresponding HTML element removing steps given removedNode and
oldParent. If removedNode is a form-associated element or the ancestor of a
form-associated element, then: If the form-associated element has a form owner and the
form-associated element and its form owner are no longer in the same
tree, then reset the form owner of the form-associated
element. If removedNode's A node is inserted into a
document when the insertion steps are invoked
with it as the argument and it is now in a document tree. Analogously, a node is removed from a document when
the removing steps are invoked with it as the
argument and it is now no longer in a document tree. A node becomes connected when the insertion steps are invoked with it as the argument and it
is now connected. Analogously, a node becomes disconnected when the removing
steps are invoked with it as the argument and it is now no longer
connected. A node is browsing-context connected when it is connected and
its shadow-including root's browsing context is non-null. A node becomes browsing-context connected when the insertion steps are invoked with it as the argument and it
is now browsing-context connected. A node becomes browsing-context disconnected either when the removing steps are invoked with it as the argument and it
is now no longer browsing-context connected, or when its shadow-including
root's browsing context becomes null.
The construction "a An IDL attribute is said to be getting when its value is being retrieved
(e.g. by author script), and is said to be setting when a new value is
assigned to it. If a DOM object is said to be live, then the attributes and methods on that object
must operate on the actual underlying data, not a snapshot of the
data. The term plugin refers to an implementation-defined set of content
handlers used by the user agent that can take part in the user agent's rendering of a
Typically such content handlers are provided by third parties, though a user agent can also
designate built-in content handlers as plugins. A user agent must not consider the types One example of a plugin would be a PDF viewer that is instantiated in a
navigable when the user navigates to a PDF file. This would count as a plugin
regardless of whether the party that implemented the PDF viewer component was the same as that
which implemented the user agent itself. However, a PDF viewer application that launches separate
from the user agent (as opposed to using the same interface) is not a plugin by this
definition. This specification does not define a mechanism for interacting with plugins, as it
is expected to be user-agent- and platform-specific. Some UAs might opt to support a plugin
mechanism such as the Netscape Plugin API; others might use remote content converters or have
built-in support for certain types. Indeed, this specification doesn't require user agents to
support plugins at all. [NPAPI] Browsers should take extreme care when interacting with external content
intended for plugins. When third-party software is run with the same
privileges as the user agent itself, vulnerabilities in the third-party software become as
dangerous as those in the user agent.
A , or just encoding where that is not ambiguous, is a defined way to convert
between byte streams and Unicode strings, as defined in Encoding. An
encoding has an and one or more , referred to as the
encoding's name and labels in the Encoding standard. [ENCODING] This specification describes the conformance criteria for user agents
(relevant to implementers) and documents (relevant to authors and
authoring tool implementers). Conforming documents are those that comply with all the conformance criteria for
documents. For readability, some of these conformance requirements are phrased as conformance
requirements on authors; such requirements are implicitly requirements on documents: by
definition, all documents are assumed to have had an author. (In some cases, that author may
itself be a user agent — such user agents are subject to additional rules, as explained
below.) For example, if a requirement states that "authors must not
use the There is no implied relationship between document conformance requirements
and implementation conformance requirements. User agents are not free to handle non-conformant
documents as they please; the processing model described in this specification applies to
implementations regardless of the conformity of the input documents. User agents fall into several (overlapping) categories with different conformance
requirements. Web browsers that support the XML syntax must process elements and attributes
from the HTML namespace found in XML documents as described in this specification,
so that users can interact with them, unless the semantics of those elements have been
overridden by other specifications. A conforming web browser would, upon finding a Web browsers that support the HTML syntax must process documents labeled with an
HTML MIME type as described in this specification, so that users can interact with
them. User agents that support scripting must also be conforming implementations of the IDL
fragments in this specification, as described in Web IDL. [WEBIDL] Unless explicitly stated, specifications that override the semantics of HTML
elements do not override the requirements on DOM objects representing those elements. For
example, the User agents that process HTML and XML documents purely to render non-interactive versions of
them must comply to the same conformance criteria as web browsers, except that they are exempt
from requirements regarding user interaction. Typical examples of non-interactive presentation user agents are printers
(static UAs) and overhead displays (dynamic UAs). It is expected that most static
non-interactive presentation user agents will also opt to lack scripting
support. A non-interactive but dynamic presentation UA would still execute scripts,
allowing forms to be dynamically submitted, and so forth. However, since the concept of "focus"
is irrelevant when the user cannot interact with the document, the UA would not need to support
any of the focus-related DOM APIs. User agents, whether interactive or not, may be designated (possibly as a user option) as
supporting the suggested default rendering defined by this specification. This is not required. In particular, even user agents that do implement the suggested default
rendering are encouraged to offer settings that override this default to improve the experience
for the user, e.g. changing the color contrast, using different focus styles, or otherwise
making the experience more accessible and usable to the user. User agents that are designated as supporting the suggested default rendering must, while so
designated, implement the rules the Rendering section defines as the
behavior that user agents are expected to implement. Implementations that do not support scripting (or which have their scripting features
disabled entirely) are exempt from supporting the events and DOM interfaces mentioned in this
specification. For the parts of this specification that are defined in terms of an events model
or in terms of the DOM, such user agents must still act as if events and the DOM were
supported. Scripting can form an integral part of an application. Web browsers that do not
support scripting, or that have scripting disabled, might be unable to fully convey the author's
intent. Conformance checkers must verify that a document conforms to the applicable conformance
criteria described in this specification. Automated conformance checkers are exempt from
detecting errors that require interpretation of the author's intent (for example, while a
document is non-conforming if the content of a Conformance checkers must check that the input document conforms when parsed without a
browsing context (meaning that no scripts are run, and
that the parser's scripting flag is disabled), and should also check that the input
document conforms when parsed with a browsing context
in which scripts execute, and that the scripts never cause non-conforming states to occur other
than transiently during script execution itself. (This is only a "SHOULD" and not a "MUST"
requirement because it has been proven to be impossible. [COMPUTABLE]) The term "HTML validator" can be used to refer to a conformance checker that itself conforms
to the applicable requirements of this specification. XML DTDs cannot express all the conformance requirements of this specification. Therefore, a
validating XML processor and a DTD cannot constitute a conformance checker. Also, since neither
of the two authoring formats defined in this specification are applications of SGML, a
validating SGML system cannot constitute a conformance checker either. To put it another way, there are three types of conformance criteria: A conformance checker must check for the first two. A simple DTD-based validator only checks
for the first class of errors and is therefore not a conforming conformance checker according
to this specification. Applications and tools that process HTML and XML documents for reasons other than to either
render the documents or check them for conformance should act in accordance with the semantics
of the documents that they process. A tool that generates document outlines but
increases the nesting level for each paragraph and does not increase the nesting level for
headings would not be conforming. Authoring tools and markup generators must generate conforming documents.
Conformance criteria that apply to authors also apply to authoring tools, where appropriate. Authoring tools are exempt from the strict requirements of using elements only for their
specified purpose, but only to the extent that authoring tools are not yet able to determine
author intent. However, authoring tools must not automatically misuse elements or encourage
their users to do so. For example, it is not conforming to use an In terms of conformance checking, an editor has to output documents that conform
to the same extent that a conformance checker will verify. When an authoring tool is used to edit a non-conforming document, it may preserve the
conformance errors in sections of the document that were not edited during the editing session
(i.e. an editing tool is allowed to round-trip erroneous content). However, an authoring tool
must not claim that the output is conformant if errors have been so preserved. Authoring tools are expected to come in two broad varieties: tools that work from structure
or semantic data, and tools that work on a What-You-See-Is-What-You-Get media-specific editing
basis (WYSIWYG). The former is the preferred mechanism for tools that author HTML, since the structure in the
source information can be used to make informed choices regarding which HTML elements and
attributes are most appropriate. However, WYSIWYG tools are legitimate. WYSIWYG tools should use elements they know are
appropriate, and should not use elements that they do not know to be appropriate. This might in
certain extreme cases mean limiting the use of flow elements to just a few elements, like
All authoring tools, whether WYSIWYG or not, should make a best effort attempt at enabling
users to create well-structured, semantically rich, media-independent content. For compatibility with existing content and prior specifications, this specification describes
two authoring formats: one based on XML, and one using a custom format inspired by SGML (referred to as the HTML syntax).
Implementations must support at least one of these two formats, although supporting both is
encouraged. Some conformance requirements are phrased as requirements on elements, attributes, methods or
objects. Such requirements fall into two categories: those describing content model restrictions,
and those describing implementation behavior. Those in the former category are requirements on
documents and authoring tools. Those in the second category are requirements on user agents.
Similarly, some conformance requirements are phrased as requirements on authors; such requirements
are to be interpreted as conformance requirements on the documents that authors produce. (In other
words, this specification does not distinguish between conformance criteria on authors and
conformance criteria on documents.) This specification relies on several other underlying specifications. The following terms are defined in Infra: [INFRA] The Unicode character set is used to represent textual data, and Encoding
defines requirements around character encodings.
[UNICODE] This specification introduces terminology
based on the terms defined in those specifications, as described earlier. The following terms are used as defined in Encoding: [ENCODING] Implementations that support the XML syntax for HTML must support some version
of XML, as well as its corresponding namespaces specification, because that syntax uses an XML
serialization with namespaces. [XML] [XMLNS] Data mining tools and other user agents that perform operations on content without running
scripts, evaluating CSS or XPath expressions, or otherwise exposing the resulting DOM to
arbitrary content, may "support namespaces" by just asserting that their DOM node analogues are
in certain namespaces, without actually exposing the namespace strings. In the HTML syntax, namespace prefixes and namespace declarations
do not have the same effect as in XML. For instance, the colon has no special meaning in HTML
element names. The attribute with the name The This specification also references the This specification also non-normatively mentions the The following terms are defined in URL: [URL] A number of schemes and protocols are referenced by this specification also: Media fragment
syntax is defined in Media Fragments URI. [MEDIAFRAG] The following terms are defined in the HTTP specifications: [HTTP] The following terms are defined in HTTP State Management Mechanism:
[COOKIES] The following term is defined in Web Linking: [WEBLINK] The following terms are defined in Structured Field Values for HTTP:
[STRUCTURED-FIELDS] The following terms are defined in MIME Sniffing: [MIMESNIFF] The following terms are defined in Fetch: [FETCH] The following terms are defined in Referrer Policy:
[REFERRERPOLICY] The following terms are defined in Mixed Content: [MIX] The following terms are defined in Subresource Integrity: [SRI] The following terms are defined in Paint Timing: [PAINTTIMING] The following terms are defined in Navigation Timing:
[NAVIGATIONTIMING] The following terms are defined in Resource Timing:
[RESOURCETIMING] The following terms are defined in Performance Timeline:
[PERFORMANCETIMELINE] The following terms are defined in Long Tasks: [LONGTASKS] The IDL fragments in this specification must be interpreted as required for conforming IDL
fragments, as described in Web IDL. [WEBIDL] The following terms are defined in Web IDL: When this specification requires a user agent to create a For instance, given the time 23045 millionths of a second after 01:00 UTC on
January 1st 2000, i.e. the time 2000-01-01T00:00:00.023045Z, then the Some parts of the language described by this specification only support JavaScript as the
underlying scripting language. [JAVASCRIPT] The term "JavaScript" is used to refer to ECMA-262, rather than the official
term ECMAScript, since the term JavaScript is more widely known. The following terms are defined in the JavaScript specification and used in this
specification: Users agents that support JavaScript must also implement ECMAScript
Internationalization API. [JSINTL] User agents that support JavaScript must also implement the Import Attributes
proposal. The following terms are defined there, and used in this specification:
[JSIMPORTATTRIBUTES] User agents that support JavaScript must also implement the JSON modules
proposal. The following terms are defined there, and used in this specification:
[JSJSONMODULES] User agents that support JavaScript must also implement the Resizable ArrayBuffer and
growable SharedArrayBuffer proposal. The following terms are defined there, and used in
this specification: [JSRESIZABLEBUFFERS] The following term is defined in WebAssembly JavaScript Interface:
[WASMJS] The Document Object Model (DOM) is a representation — a model — of a document and
its content. The DOM is not just an API; the conformance criteria of HTML implementations are
defined, in this specification, in terms of operations on the DOM. [DOM] Implementations must support DOM and the events defined in UI Events, because this
specification is defined in terms of the DOM, and some of the features are defined as extensions
to the DOM interfaces. [DOM] [UIEVENTS] In particular, the following features are defined in DOM: [DOM] The following features are defined in UI Events: [UIEVENTS] The following features are defined in Touch Events: [TOUCH] The following features are defined in Pointer Events:
[POINTEREVENTS] The following events are defined in Clipboard API and events:
[CLIPBOARD-APIS] This specification sometimes uses the term name to refer to the event's
type; as in, "an event named The following features are defined in DOM Parsing and Serialization:
[DOMPARSING] The following features are defined in Selection API: [SELECTION] User agents are encouraged to implement the features described in
execCommand. [EXECCOMMAND] The following parts of Fullscreen API are referenced from this
specification, in part to define the rendering of High Resolution Time provides the current high
resolution time, the unsafe shared
current time, the shared monotonic clock,
the coarsen time
algorithm, and the This specification uses the following features defined in File API:
[FILEAPI] This specification uses cleanup
Indexed Database transactions defined by Indexed Database API.
[INDEXEDDB] The following terms are defined in Media Source Extensions:
[MEDIASOURCE] The following terms are defined in Media Capture and Streams:
[MEDIASTREAM] The following terms are defined in Reporting: [REPORTING] The following features and terms are defined in XMLHttpRequest:
[XHR] The following features are defined in Battery Status API: [BATTERY] Implementations must support Media Queries. The <media-condition>
feature is defined therein. [MQ] While support for CSS as a whole is not required of implementations of this specification
(though it is encouraged, at least for web browsers), some features are defined in terms of
specific CSS requirements. When this specification requires that something be parsed according
to a particular CSS grammar, the relevant algorithm in CSS Syntax must be
followed, including error handling rules. [CSSSYNTAX] For example, user agents are required to close all open constructs upon
finding the end of a style sheet unexpectedly. Thus, when parsing the string " To parse a CSS <color> value, given a string
input with an optional element element, run these steps: Let color be the result of parsing input as a CSS <color>.
[CSSCOLOR] If color is failure, then return failure. If color is 'currentcolor', then: If element is not given, then set color to opaque
black. Otherwise, set color to the computed value of the 'color'
property of element. Return color. The following terms and features are defined in Cascading Style Sheets
(CSS): [CSS] The basic version of the 'display' property
is defined in CSS, and the property is extended by other CSS modules.
[CSS] [CSSRUBY] [CSSTABLE] The following terms and features are defined in CSS Box Model:
[CSSBOX] The following features are defined in CSS Logical Properties:
[CSSLOGICAL] The following terms and features are defined in CSS Color:
[CSSCOLOR] The following terms are defined in CSS Images: [CSSIMAGES] The term paint
source is used as defined in CSS Images Level 4
to define the interaction of certain HTML elements with the CSS 'element()' function.
[CSSIMAGES4] The following features are defined in CSS Backgrounds and Borders:
[CSSBG] CSS Backgrounds and Borders also defines the following border properties:
[CSSBG] The following features are defined in CSS Box Alignment: [CSSALIGN] The following terms and features are defined in CSS Display:
[CSSDISPLAY] The following features are defined in CSS Flexible Box Layout:
[CSSFLEXBOX] The following terms and features are defined in CSS Fonts:
[CSSFONTS] The following features are defined in CSS Grid Layout: [CSSGRID] The following terms are defined in CSS Inline Layout: [CSSINLINE] The following terms and features are defined in CSS Box Sizing:
[CSSSIZING] The following features are defined in CSS Lists and Counters.
[CSSLISTS] The following features are defined in CSS Overflow. [CSSOVERFLOW] The following terms and features are defined in CSS Positioned Layout:
[CSSPOSITION] The following features are defined in CSS Multi-column Layout.
[CSSMULTICOL] The 'ruby-base'
value of the 'display' property is defined in CSS Ruby Layout.
[CSSRUBY] The following features are defined in CSS Table: [CSSTABLE] The following features are defined in CSS Text: [CSSTEXT] The following features are defined in CSS Writing Modes: [CSSWM] The following features are defined in CSS Basic User Interface:
[CSSUI] The algorithm to update
animations and send events is defined in Web Animations.
[WEBANIMATIONS]. Implementations that support scripting must support the CSS Object Model. The following
features and terms are defined in the CSSOM specifications: [CSSOM]
[CSSOMVIEW]
The following features and terms are defined in CSS Syntax:
[CSSSYNTAX] The following terms are defined in Selectors: [SELECTORS] The following features are defined in CSS Values and Units:
[CSSVALUES] The following features are defined in CSS View Transitions 1:
[CSSVIEWTRANSITIONS] The term style attribute is
defined in CSS Style Attributes. [CSSATTR] The following terms are defined in the CSS Cascading and Inheritance:
[CSSCASCADE] The The following interfaces and terms are defined in Geometry Interfaces:
[GEOMETRY] The following terms are defined in the CSS Scoping: [CSSSCOPING] The following terms and features are defined in CSS Color Adjustment:
[CSSCOLORADJUST] The following term is defined in CSS Pseudo-Elements: [CSSPSEUDO] The following terms are defined in CSS Containment: [CSSCONTAIN] The following term is defined in Intersection Observer:
[INTERSECTIONOBSERVER] The following terms are defined in Resize Observer:
[RESIZEOBSERVER] The following interfaces are defined in the WebGL specifications: [WEBGL] The following interfaces are defined in WebGPU: [WEBGPU] Implementations may support WebVTT as a text track format for subtitles, captions, metadata,
etc., for media resources. [WEBVTT] The following terms, used in this specification, are defined in WebVTT: The In addition, the following Finally, the following terms are defined ARIA: [ARIA] The following terms are defined in Content Security Policy: [CSP] The following terms are defined in Service Workers: [SW] The following algorithms are defined in Secure Contexts:
[SECURE-CONTEXTS] The following terms are defined in Permissions Policy:
[PERMISSIONSPOLICY] The following feature is defined in Payment Request API:
[PAYMENTREQUEST] While support for MathML as a whole is not required by this specification (though it is
encouraged, at least for web browsers), certain features depend upon small parts of MathML being
implemented. [MATHML] The following features are defined in Mathematical Markup Language
(MathML): While support for SVG as a whole is not required by this specification (though it is
encouraged, at least for web browsers), certain features depend upon parts of SVG being
implemented. User agents that implement SVG must implement the SVG 2 specification, and not
any earlier revisions. The following features are defined in the SVG 2 specification:
[SVG] The following features are defined in Filter Effects: [FILTERS] The following features are defined in Compositing and Blending:
[COMPOSITE] The following features are defined in Cooperative Scheduling of Background
Tasks: [REQUESTIDLECALLBACK] The following terms are defined in Screen Orientation:
[SCREENORIENTATION] The following terms are defined in Storage: [STORAGE] The following features are defined in Web App Manifest: [MANIFEST] The following features are defined in WebCodecs: [WEBCODECS] The following terms are defined in WebDriver: [WEBDRIVER] The following terms are defined in WebDriver BiDi: [WEBDRIVERBIDI] The following terms are defined in Web Cryptography API:
[WEBCRYPTO] The following terms are defined in WebSockets: [WEBSOCKETS] The following terms are defined in WebTransport: [WEBTRANSPORT] The following terms are defined in Web Authentication: An API for accessing Public Key
Credentials: [WEBAUTHN] The following terms are defined in Credential Management: [CREDMAN] The following terms are defined in Console: [CONSOLE] This specification does not require support of any particular network protocol, style
sheet language, scripting language, or any of the DOM specifications beyond those required in the
list above. However, the language described by this specification is biased towards CSS as the
styling language, JavaScript as the scripting language, and HTTP as the network protocol, and
several features assume that those languages and protocols are in use. A user agent that implements the HTTP protocol must implement HTTP State Management
Mechanism (Cookies) as well. [HTTP] [COOKIES] This specification might have certain additional requirements on character
encodings, image formats, audio formats, and video formats in the respective sections. Vendor-specific proprietary user agent extensions to this specification are strongly
discouraged. Documents must not use such extensions, as doing so reduces interoperability and
fragments the user base, allowing only users of specific user agents to access the content in
question. All extensions must be defined so that the use of extensions neither contradicts nor causes the
non-conformance of functionality defined in the specification. For example, while strongly discouraged from doing so, an implementation could add a new IDL
attribute " When vendor-neutral extensions to this specification are needed, either this specification can
be updated accordingly, or an extension specification can be written that overrides the
requirements in this specification. When someone applying this specification to their activities
decides that they will recognize the requirements of such an extension specification, it becomes
an applicable specification for the purposes
of conformance requirements in this specification. Someone could write a specification that defines any arbitrary byte stream as
conforming, and then claim that their random junk is conforming. However, that does not mean that
their random junk actually is conforming for everyone's purposes: if someone else decides that
that specification does not apply to their work, then they can quite legitimately say that the
aforementioned random junk is just that, junk, and not conforming at all. As far as conformance
goes, what matters in a particular community is what that community agrees is
applicable. User agents must treat elements and attributes that they do not understand as semantically
neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS
processors), but not inferring any meaning from them. When support for a feature is disabled (e.g. as an emergency measure to mitigate a security
problem, or to aid in development, or for performance reasons), user agents must act as if they
had no support for the feature whatsoever, and as if the feature was not mentioned in this
specification. For example, if a particular feature is accessed via an attribute in a Web IDL
interface, the attribute itself would be omitted from the objects that implement that interface
— leaving the attribute on the object but making it return null or throw an exception is
insufficient. Implementations of XPath 1.0 that operate on HTML
documents parsed or created in the manners described in this specification (e.g. as part of
the First, remove this paragraph: A QName in the node test is expanded
into an expanded-name
using the namespace declarations from the expression context. This is the same way expansion is
done for element type names in start and end-tags except that the default namespace declared with
Then, insert in its place the following: A QName in the node test is expanded into an expanded-name using the namespace declarations
from the expression context. If the QName has a prefix, then there must be a namespace declaration for this prefix in
the expression context, and the corresponding namespace URI is the one that is
associated with this prefix. It is an error if the QName has a prefix for which there is no
namespace declaration in the expression context. If the QName has no prefix and the principal node type of the axis is element, then the
default element namespace is used. Otherwise if the QName has no prefix, the namespace URI is
null. The default element namespace is a member of the context for the XPath expression. The
value of the default element namespace when executing an XPath expression through the DOM3 XPath
API is determined in the following way: This is equivalent to adding the default element namespace feature of XPath 2.0
to XPath 1.0, and using the HTML namespace as the default element namespace for HTML documents.
It is motivated by the desire to have implementations be compatible with legacy HTML content
while still supporting the changes that this specification introduces to HTML regarding the
namespace used for HTML elements, and by the desire to use XPath 1.0 rather than XPath 2.0. This change is a willful violation of the XPath 1.0 specification,
motivated by desire to have implementations be compatible with legacy content while still
supporting the changes that this specification introduces to HTML regarding which namespace is
used for HTML elements. [XPATH10] XSLT 1.0 processors outputting to a DOM when the output
method is "html" (either explicitly or via the defaulting rule in XSLT 1.0) are affected as
follows: If the transformation program outputs an element in no namespace, the processor must, prior to
constructing the corresponding DOM element node, change the namespace of the element to the
HTML namespace, ASCII-lowercase the
element's local name, and ASCII-lowercase the
names of any non-namespaced attributes on the element. This requirement is a willful violation of the XSLT 1.0
specification, required because this specification changes the namespaces and case-sensitivity
rules of HTML in a manner that would otherwise be incompatible with DOM-based XSLT
transformations. (Processors that serialize the output are unaffected.) [XSLT10] This specification does not specify precisely how XSLT processing interacts with the HTML
parser infrastructure (for example, whether an XSLT processor acts as if it puts any
elements into a stack of open elements). However, XSLT processors must stop
parsing if they successfully complete, and must update the current document
readiness first to " This specification does not specify how XSLT interacts with the navigation algorithm, how it fits in with the event loop, nor
how error pages are to be handled (e.g. whether XSLT errors are to replace an incremental XSLT
output, or are rendered inline, etc.). There are also additional non-normative comments regarding the interaction of XSLT
and HTML in the Headers/Permissions-Policy/document-domain Support in one engine only. This document defines the following policy-controlled features: Headers/Feature-Policy/autoplay Headers/Permissions-Policy/autoplay Support in one engine only. There are various places in HTML that accept particular data types, such as dates or numbers.
This section describes what the conformance criteria for content in those formats is, and how to
parse them. Implementers are strongly urged to carefully examine any third-party libraries
they might consider using to implement the parsing of syntaxes described below. For example, date
libraries are likely to implement error handling behavior that differs from what is required in
this specification, since error-handling behavior is often not defined in specifications that
describe date syntaxes similar to those used in this specification, and thus implementations tend
to vary greatly in how they handle errors. Some of the micro-parsers described below follow the pattern of having an input
variable that holds the string being parsed, and having a position variable pointing at
the next character to parse in input. A number of attributes are boolean attributes. The
presence of a boolean attribute on an element represents the true value, and the absence of the
attribute represents the false value. If the attribute is present, its value must either be the empty string or a value that is an
ASCII case-insensitive match for the attribute's canonical name, with no leading or
trailing whitespace. The values "true" and "false" are not allowed on boolean attributes. To represent
a false value, the attribute has to be omitted altogether. Here is an example of a checkbox that is checked and disabled. The This could be equivalently written as this:
You can also mix styles; the following is still equivalent: Some attributes, called enumerated attributes, take on a finite set of states. The state for such an
attribute is derived by combining the attribute's value, a set of keyword/state mappings given in
the specification of each attribute, and two possible special states that can also be given in the
specification of the attribute. These special states are the invalid value
default and the missing value default. Multiple keywords can map to the same state. The empty string can be a valid keyword. Note that the missing value default applies only when the attribute is missing, not when
it is present with an empty string value. To determine the state of an attribute, use the following steps: If the attribute is not specified: If the attribute has a missing value default
state defined, then return that missing value default
state. Otherwise, return no state. If the attribute's value is an ASCII case-insensitive match for one of the
keywords defined for the attribute, then return the state represented by that keyword. If the attribute has an invalid value default state
defined, then return that invalid value default
state. Return no state. For authoring conformance purposes, if an enumerated attribute is specified, the attribute's
value must be an ASCII case-insensitive match for one of the conforming keywords for
that attribute, with no leading or trailing whitespace. For reflection purposes, states which have any keywords mapping
to them are said to have a canonical keyword. This is determined as follows: If there is only one keyword mapping to the given state, then it is that keyword. If there is only one conforming keyword mapping to the given state, then it is
that conforming keyword. Otherwise, the canonical keyword for the state will be explicitly given in the
specification for the attribute. A string is a valid integer if it consists of one or more ASCII digits,
optionally prefixed with a U+002D HYPHEN-MINUS character (-). A valid integer without a U+002D HYPHEN-MINUS (-) prefix represents the number
that is represented in base ten by that string of digits. A valid integer
with a U+002D HYPHEN-MINUS (-) prefix represents the number represented in base ten by
the string of digits that follows the U+002D HYPHEN-MINUS, subtracted from zero. The rules for parsing integers are as given in the following algorithm. When
invoked, the steps must be followed in the order given, aborting at the first step that returns a
value. This algorithm will return either an integer or an error. Let input be the string being parsed. Let position be a pointer into input, initially pointing at the
start of the string. Let sign have the value "positive". Skip ASCII whitespace within input given
position. If position is past the end of input, return an error. If the character indicated by position (the first character) is a U+002D
HYPHEN-MINUS character (-): Otherwise, if the character indicated by position (the first character) is a
U+002B PLUS SIGN character (+): If the character indicated by position is not an ASCII digit, then return an error. Collect a sequence of code points that are ASCII digits from
input given position, and interpret the resulting sequence as a base-ten
integer. Let value be that integer. If sign is "positive", return value, otherwise return the result of
subtracting value from zero. A string is a valid non-negative integer if it consists of one or more ASCII
digits. A valid non-negative integer represents the number that is represented in base ten
by that string of digits. The rules for parsing non-negative integers are as given in the following algorithm.
When invoked, the steps must be followed in the order given, aborting at the first step that
returns a value. This algorithm will return either zero, a positive integer, or an error. Let input be the string being parsed. Let value be the result of parsing input using the
rules for parsing integers. If value is an error, return an error. If value is less than zero, return an error. Return value. A string is a valid floating-point number if it consists of: A valid floating-point number represents the number obtained by multiplying the
significand by ten raised to the power of the exponent, where the significand is the first number,
interpreted as base ten (including the decimal point and the number after the decimal point, if
any, and interpreting the significand as a negative number if the whole string starts with a
U+002D HYPHEN-MINUS character (-) and the number is not zero), and where the exponent is the
number after the E, if any (interpreted as a negative number if there is a U+002D HYPHEN-MINUS
character (-) between the E and the number and the number is not zero, or else ignoring a U+002B
PLUS SIGN character (+) between the E and the number if there is one). If there is no E, then the
exponent is treated as zero. The Infinity and Not-a-Number (NaN) values are not valid floating-point numbers. The valid floating-point number concept is typically only used to
restrict what is allowed for authors, while the user agent requirements use the rules for
parsing floating-point number values below (e.g., the The best
representation of the number n as a floating-point number is the string
obtained from running ToString(n). The abstract operation
ToString is not uniquely determined. When there are multiple possible strings that
could be obtained from ToString for a particular value, the user agent must always
return the same string for that value (though it may differ from the value used by other user
agents). The rules for parsing floating-point number values are as given in the
following algorithm. This algorithm must be aborted at the first step that returns something.
This algorithm will return either a number or an error. Let input be the string being parsed. Let position be a pointer into input, initially pointing at the
start of the string. Let value have the value 1. Let divisor have the value 1. Let exponent have the value 1. Skip ASCII whitespace within input given
position. If position is past the end of input, return an error. If the character indicated by position is a U+002D HYPHEN-MINUS character (-): Otherwise, if the character indicated by position (the first character) is a
U+002B PLUS SIGN character (+): If the character indicated by position is a U+002E FULL STOP (.), and that is
not the last character in input, and the character after the character indicated by
position is an ASCII digit, then set
value to zero and jump to the step labeled fraction. If the character indicated by position is not an ASCII digit, then return an error. Collect a sequence of code points that are ASCII digits from
input given position, and interpret the resulting sequence as a base-ten
integer. Multiply value by that integer. Fraction: If the character indicated by position is a U+002E
FULL STOP (.), run these substeps: Advance position to the next character. If position is past the end of input, or if the character
indicated by position is not an ASCII digit,
U+0065 LATIN SMALL LETTER E (e), or U+0045 LATIN CAPITAL LETTER E (E), then jump to the step
labeled conversion. If the character indicated by position is a U+0065 LATIN SMALL
LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E), skip the remainder of
these substeps. Fraction loop: Multiply divisor by ten. Advance position to the next character. If position is past the end of input, then jump to the step
labeled conversion. If the character indicated by position is an ASCII digit, jump back to the step labeled fraction loop in these
substeps. If the character indicated by position is U+0065 (e) or a U+0045 (E), then: Advance position to the next character. If position is past the end of input, then jump to the step
labeled conversion. If the character indicated by position is a U+002D HYPHEN-MINUS character
(-): If position is past the end of input, then jump to the step
labeled conversion. Otherwise, if the character indicated by position is a U+002B PLUS SIGN
character (+): If position is past the end of input, then jump to the step
labeled conversion. If the character indicated by position is not an ASCII digit, then jump to the step labeled conversion. Collect a sequence of code points that are ASCII digits from
input given position, and interpret the resulting sequence as a base-ten
integer. Multiply exponent by that integer. Multiply value by ten raised to the exponentth power. Conversion: Let S be the set of finite IEEE 754 double-precision
floating-point values except −0, but with two special values added: 21024 and −21024. Let rounded-value be the number in S that is closest to
value, selecting the number with an even significand if there are two equally close
values. (The two special values 21024 and −21024
are considered to have even significands for this purpose.) If rounded-value is 21024 or −21024, return an error. Return rounded-value. The rules for parsing dimension values are as given in the following algorithm. When
invoked, the steps must be followed in the order given, aborting at the first step that returns a
value. This algorithm will return either a number greater than or equal to 0.0, or failure; if a
number is returned, then it is further categorized as either a percentage or a length. Let input be the string being parsed. Let position be a position variable for input,
initially pointing at the start of input. Skip ASCII whitespace within input given
position. If position is past the end of input or the code point at
position within input is not an ASCII
digit, then return failure. Collect a sequence of code points that are ASCII digits from
input given position, and interpret the resulting sequence as a base-ten
integer. Let value be that number. If position is past the end of input, then return value
as a length. If the code point at position within input is U+002E (.), then: Advance position by 1. If position is past the end of input or the code point at
position within input is not an ASCII
digit, then return the current dimension value with value,
input, and position. Let divisor have the value 1. While true: Multiply divisor by ten. Add the value of the code point at position within input,
interpreted as a base-ten digit (0..9) and divided by divisor, to
value. Advance position by 1. If position is past the end of input, then return
value as a length. If the code point at position within input is not an ASCII digit, then break. Return the current dimension value with value, input,
and position. The current dimension value, given value, input, and
position, is determined as follows: If position is past the end of input, then return value
as a length. If the code point at position within input is U+0025 (%), then
return value as a percentage. Return value as a length. The rules for parsing nonzero dimension
values are as given in the following algorithm. When invoked, the steps must be followed in
the order given, aborting at the first step that returns a value. This algorithm will return
either a number greater than 0.0, or an error; if a number is returned, then it is further
categorized as either a percentage or a length. Let input be the string being parsed. Let value be the result of parsing input using the rules for
parsing dimension values. If value is an error, return an error. If value is zero, return an error. If value is a percentage, return value as a percentage. Return value as a length. A valid list of floating-point numbers is a number of valid floating-point numbers separated by U+002C COMMA characters,
with no other characters (e.g. no ASCII whitespace). In addition, there might be
restrictions on the number of floating-point numbers that can be given, or on the range of values
allowed. The rules for parsing a list of floating-point numbers are as follows: Let input be the string being parsed. Let position be a pointer into input, initially pointing at the
start of the string. Let numbers be an initially empty list of floating-point numbers. This list
will be the result of this algorithm. Collect a sequence of code points that are ASCII whitespace,
U+002C COMMA, or U+003B SEMICOLON characters from input given position.
This skips past any leading delimiters. While position is not past the end of input: Collect a sequence of code points that are not ASCII
whitespace, U+002C COMMA, U+003B SEMICOLON, ASCII digits, U+002E FULL STOP,
or U+002D HYPHEN-MINUS characters from input given position. This skips
past leading garbage. Collect a sequence of code points that are not ASCII
whitespace, U+002C COMMA, or U+003B SEMICOLON characters from input given
position, and let unparsed number be the result. Let number be the result of parsing unparsed number using the
rules for parsing floating-point number values. If number is an error, set number to zero. Append number to numbers. Collect a sequence of code points that are ASCII whitespace,
U+002C COMMA, or U+003B SEMICOLON characters from input given position.
This skips past the delimiter. Return numbers. The rules for parsing a list of dimensions are as follows. These rules return a list
of zero or more pairs consisting of a number and a unit, the unit being one of percentage,
relative, and absolute. Let raw input be the string being parsed. If the last character in raw input is a U+002C COMMA character (,),
then remove that character from raw input. Split the string raw input on
commas. Let raw tokens be the resulting list of tokens. Let result be an empty list of number/unit pairs. For each token in raw tokens, run the following substeps: Let input be the token. Let position be a pointer into input,
initially pointing at the start of the string. Let value be the number 0. Let unit be absolute. If position is past the end of input, set unit to relative and jump to the last substep. If the character at position is an ASCII digit, collect a sequence of code points that are ASCII
digits from input given position, interpret the resulting sequence
as an integer in base ten, and increment value by that integer. If the character at position is U+002E (.), then: Collect a sequence of code points consisting of ASCII
whitespace and ASCII digits from input given
position. Let s be the resulting sequence. Remove all ASCII whitespace in s. If s is not the empty string, then: Let length be the number of characters in s (after the spaces were removed). Let fraction be the result of interpreting s as a base-ten integer, and then dividing that number by 10length. Increment value by fraction. Skip ASCII whitespace within input given
position. If the character at position is a U+0025 PERCENT SIGN character (%),
then set unit to percentage. Otherwise, if the character at position is a U+002A ASTERISK character
(*), then set unit to relative. Add an entry to result consisting of the number given by value and the unit given by unit. Return the list result. In the algorithms below, the number of days in month month of year
year is: 31 if month is 1, 3, 5, 7, 8,
10, or 12; 30 if month is 4, 6, 9, or 11; 29 if month is 2
and year is a number divisible by 400, or if year is a number divisible by 4
but not by 100; and 28 otherwise. This takes into account leap years in the Gregorian
calendar. [GREGORIAN] When ASCII digits are used in the date and time syntaxes defined in this section,
they express numbers in base ten. While the formats described here are intended to be subsets of the corresponding
ISO8601 formats, this specification defines parsing rules in much more detail than ISO8601.
Implementers are therefore encouraged to carefully examine any date parsing libraries before using
them to implement the parsing rules described below; ISO8601 libraries might not parse dates and
times in exactly the same manner. [ISO8601] Where this specification refers to the proleptic Gregorian calendar, it means the
modern Gregorian calendar, extrapolated backwards to year 1. A date in the proleptic
Gregorian calendar, sometimes explicitly referred to as a proleptic-Gregorian
date, is one that is described using that calendar even if that calendar was not in use at
the time (or place) in question. [GREGORIAN] The use of the Gregorian calendar as the wire format in this specification is an
arbitrary choice resulting from the cultural biases of those involved in the decision. See also
the section discussing date, time, and number formats in forms
(for authors), implementation notes regarding
localization of form controls, and the A month consists of a specific proleptic-Gregorian
date with no time-zone information and no date information beyond a year and a month.
[GREGORIAN] A string is a valid month string representing a year year and
month month if it consists of the following components in the given order: The rules to parse a month string are as follows. This will return either a year and
month, or nothing. If at any point the algorithm says that it "fails", this means that it is
aborted at that point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Parse a month component to obtain year and month. If this returns nothing, then fail. If position is not beyond the
end of input, then fail. Return year and month. The rules to parse a month component, given an input string and
a position, are as follows. This will return either a year and a month, or
nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that
point and returns nothing. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not at least four
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the year. If year is not a number greater than zero, then fail. If position is beyond the end of input or if the
character at position is not a U+002D HYPHEN-MINUS character, then fail.
Otherwise, move position forwards one character. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not exactly two
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the month. If month is not a number in the range 1 ≤ month ≤ 12, then fail. Return year and month. A date consists of a specific proleptic-Gregorian
date with no time-zone information, consisting of a year, a month, and a day.
[GREGORIAN] A string is a valid date string representing a year year, month
month, and day day if it consists of the following
components in the given order: The rules to parse a date string are as follows. This will return either a date, or
nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that
point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Parse a date component to obtain year, month, and day. If this returns nothing, then fail. If position is not beyond the end of input, then fail. Let date be the date with year year, month month, and day day. Return date. The rules to parse a date component, given an input string and a
position, are as follows. This will return either a year, a month, and a day,
or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at
that point and returns nothing. Parse a month component to obtain year and month. If this returns nothing, then fail. Let maxday be the number of days in month month of year year. If position is beyond the end of input or if the
character at position is not a U+002D HYPHEN-MINUS character, then fail.
Otherwise, move position forwards one character. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not exactly two
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the day. If day is not a number in the range 1 ≤ day ≤ maxday, then fail. Return year, month, and day. A yearless date consists of a Gregorian month and a
day within that month, but with no associated year. [GREGORIAN] A string is a valid yearless date string representing a month month and a day day if it consists of the following components
in the given order: In other words, if the month is " The rules to parse a yearless date string are as follows. This will return either a
month and a day, or nothing. If at any point the algorithm says that it "fails", this means that
it is aborted at that point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Parse a yearless date component to obtain month and day. If this returns nothing, then fail. If position is not beyond the end of input, then fail. Return month and day. The rules to parse a yearless date component, given an input
string and a position, are as follows. This will return either a month and a
day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted
at that point and returns nothing. Collect a sequence of code points that are U+002D HYPHEN-MINUS characters (-)
from input given position. If the collected sequence is not exactly zero or
two characters long, then fail. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not exactly two
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the month. If month is not a number in the range 1 ≤ month ≤ 12, then fail. Let maxday be the number of days in month month of any arbitrary leap year (e.g. 4
or 2000). If position is beyond the end of input or if the
character at position is not a U+002D HYPHEN-MINUS character, then fail.
Otherwise, move position forwards one character. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not exactly two
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the day. If day is not a number in the range 1 ≤ day ≤ maxday, then fail. Return month and day. A time consists of a specific time with no time-zone
information, consisting of an hour, a minute, a second, and a fraction of a second. A string is a valid time string representing an hour hour, a
minute minute, and a second second if it consists of the
following components in the given order: The second component cannot be 60 or 61; leap seconds cannot
be represented. The rules to parse a time string are as follows. This will return either a time, or
nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that
point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail. If position is not beyond the end of input, then fail. Let time be the time with hour hour, minute minute, and second second. Return time. The rules to parse a time component, given an input string and a
position, are as follows. This will return either an hour, a minute, and a
second, or nothing. If at any point the algorithm says that it "fails", this means that it is
aborted at that point and returns nothing. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not exactly two
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the hour. If position is beyond the end of input or if the
character at position is not a U+003A COLON character, then fail. Otherwise,
move position forwards one character. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not exactly two
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the minute. Let second be 0. If position is not beyond the end of input and the character at
position is U+003A (:), then: Advance position to the next character in input. If position is beyond the end of input, or at
the last character in input, or if the next two characters in input starting at position are not both ASCII
digits, then fail. Collect a sequence of code points that are either ASCII digits
or U+002E FULL STOP characters from input given position. If the
collected sequence is three characters long, or if it is longer than three characters long and
the third character is not a U+002E FULL STOP character, or if it has more than one U+002E FULL
STOP character, then fail. Otherwise, interpret the resulting sequence as a base-ten number
(possibly with a fractional part). Set second to that number. If second is not a number in the range
0 ≤ second < 60, then fail. Return hour, minute, and second. A local date and time consists of a specific
proleptic-Gregorian date, consisting of a year, a month, and a day, and a time,
consisting of an hour, a minute, a second, and a fraction of a second, but expressed without a
time zone. [GREGORIAN] A string is a valid local date and time string representing a date and time if it
consists of the following components in the given order: A string is a valid
normalized local date and time string representing a date and time if it consists of the
following components in the given order: The rules to parse a local date and time string are as follows. This will return
either a date and time, or nothing. If at any point the algorithm says that it "fails", this means
that it is aborted at that point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Parse a date component to obtain year, month, and day. If this returns nothing, then fail. If position is beyond the end of input or if the
character at position is neither a U+0054 LATIN CAPITAL LETTER T character
(T) nor a U+0020 SPACE character, then fail. Otherwise, move position
forwards one character. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail. If position is not beyond the end of input, then fail. Let date be the date with year year, month month, and day day. Let time be the time with hour hour, minute minute, and second second. Return date and time. A time-zone offset consists of a signed number of hours and
minutes. A string is a valid time-zone offset string representing a time-zone offset if it
consists of either: A U+005A LATIN CAPITAL LETTER Z character (Z), allowed only if the time zone is
UTC Or, the following components, in the given order: This format allows for time-zone offsets from -23:59 to +23:59. Right now, in
practice, the range of offsets of actual time zones is -12:00 to +14:00, and the minutes component
of offsets of actual time zones is always either 00, 30, or 45. There is no guarantee that this
will remain so forever, however, since time zones are used as political footballs and are thus
subject to very whimsical policy decisions. See also the usage notes and examples in the global
date and time section below for details on using time-zone offsets with historical times
that predate the formation of formal time zones. The rules to parse a time-zone offset string are as follows. This will return either
a time-zone offset, or nothing. If at any point the algorithm says that it "fails", this means
that it is aborted at that point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this
returns nothing, then fail. If position is not beyond the end of input, then fail. Return the time-zone offset that is timezonehours
hours and timezoneminutes minutes from UTC. The rules to parse a time-zone offset component, given an input
string and a position, are as follows. This will return either time-zone hours
and time-zone minutes, or nothing. If at any point the algorithm says that it "fails", this means
that it is aborted at that point and returns nothing. If the character at position is a U+005A LATIN CAPITAL LETTER Z character
(Z), then: Let timezonehours be 0. Let timezoneminutes be 0. Advance position to the next character in input. Otherwise, if the character at position is either a U+002B PLUS SIGN (+)
or a U+002D HYPHEN-MINUS (-), then: If the character at position is a U+002B PLUS SIGN (+), let sign be "positive". Otherwise, it's a U+002D HYPHEN-MINUS (-); let sign be "negative". Advance position to the next character in input. Collect a sequence of code points that are ASCII digits from
input given position. Let s be the collected
sequence. If s is exactly two characters long, then: Interpret s as a base-ten integer. Let that number be the timezonehours. If position is beyond the end of input or if
the character at position is not a U+003A COLON character, then fail.
Otherwise, move position forwards one character. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not exactly two
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten
integer. Let that number be the timezoneminutes. If s is exactly four characters long, then: Interpret the first two characters of s as a base-ten integer. Let
that number be the timezonehours. Interpret the last two characters of s as a base-ten integer. Let
that number be the timezoneminutes. Otherwise, fail. Otherwise, fail. Return timezonehours and timezoneminutes. A global date and time consists of a specific
proleptic-Gregorian date, consisting of a year, a month, and a day, and a time,
consisting of an hour, a minute, a second, and a fraction of a second, expressed with a time-zone
offset, consisting of a signed number of hours and minutes. [GREGORIAN] A string is a valid global date and time string representing a date, time, and a
time-zone offset if it consists of the following components in the given order: Times in dates before the formation of UTC in the mid-twentieth century must be expressed and
interpreted in terms of UT1 (contemporary Earth solar time at the 0° longitude), not UTC (the
approximation of UT1 that ticks in SI seconds). Time before the formation of time zones must be
expressed and interpreted as UT1 times with explicit time zones that approximate the contemporary
difference between the appropriate local time and the time observed at the location of Greenwich,
London. The following are some examples of dates written as valid global date and time strings. Several things are notable about these dates: The rules to parse a global date and time string are as follows. This will return
either a time in UTC, with associated time-zone offset information for round-tripping or display
purposes, or nothing. If at any point the algorithm says that it "fails", this means that it is
aborted at that point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Parse a date component to obtain year, month, and day. If this returns nothing, then fail. If position is beyond the end of input or if the
character at position is neither a U+0054 LATIN CAPITAL LETTER T character
(T) nor a U+0020 SPACE character, then fail. Otherwise, move position
forwards one character. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail. If position is beyond the end of input, then
fail. Parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this
returns nothing, then fail. If position is not beyond the end of input, then fail. Let time be the moment in time at year year, month
month, day day, hours hour, minute
minute, second second, subtracting timezonehours hours and timezoneminutes minutes. That moment in time is a moment in the UTC time
zone. Let timezone be timezonehours
hours and timezoneminutes minutes from UTC. Return time and timezone. A week consists of a week-year number and a week number
representing a seven-day period starting on a Monday. Each week-year in this calendaring system
has either 52 or 53 such seven-day periods, as defined below. The seven-day period starting on the
Gregorian date Monday December 29th 1969 (1969-12-29) is defined as week number 1 in week-year
1970. Consecutive weeks are numbered sequentially. The week before the number 1 week in a
week-year is the last week in the previous week-year, and vice versa. [GREGORIAN] A week-year with a number year has 53 weeks if it corresponds to either a
year year in the proleptic Gregorian calendar that has a Thursday
as its first day (January 1st), or a year year in the proleptic
Gregorian calendar that has a Wednesday as its first day (January 1st) and where year is a number divisible by 400, or a number divisible by 4 but not by 100. All
other week-years have 52 weeks. The week number of the last day of a week-year with 53 weeks is 53; the week number
of the last day of a week-year with 52 weeks is 52. The week-year number of a particular day can be different than the number of the
year that contains that day in the proleptic Gregorian calendar. The first week in a
week-year y is the week that contains the first Thursday of the Gregorian year
y. For modern purposes, a week as defined here is
equivalent to ISO weeks as defined in ISO 8601. [ISO8601] A string is a valid week string representing a week-year year
and week week if it consists of the following components in the given
order: The rules to parse a week string are as follows. This will return either a week-year
number and week number, or nothing. If at any point the algorithm says that it "fails", this means
that it is aborted at that point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not at least four
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the year. If year is not a number greater than zero, then fail. If position is beyond the end of input or if the
character at position is not a U+002D HYPHEN-MINUS character, then fail.
Otherwise, move position forwards one character. If position is beyond the end of input or if the
character at position is not a U+0057 LATIN CAPITAL LETTER W character (W),
then fail. Otherwise, move position forwards one character. Collect a sequence of code points that are ASCII digits from
input given position. If the collected sequence is not exactly two
characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
Let that number be the week. Let maxweek be the week number of the last day of year
year. If week is not a number in the range 1 ≤ week ≤ maxweek, then fail. If position is not beyond the end of input, then fail. Return the week-year number year and the week number week. A duration consists of a number of seconds. Since months and seconds are not comparable (a month is not a precise number of
seconds, but is instead a period whose exact length depends on the precise day from which it is
measured) a duration as defined in this specification cannot
include months (or years, which are equivalent to
twelve months). Only durations that describe a specific number of seconds can be described. A string is a valid duration string representing a duration t if it consists of either of the
following: A literal U+0050 LATIN CAPITAL LETTER P character followed by one or more of the following
subcomponents, in the order given, where the number of days, hours, minutes, and
seconds corresponds to the same number of seconds as in t: One or more ASCII digits followed by a U+0044 LATIN CAPITAL LETTER D
character, representing a number of days. A U+0054 LATIN CAPITAL LETTER T character followed by one or more of the following
subcomponents, in the order given: One or more ASCII digits followed by a U+0048 LATIN CAPITAL LETTER H
character, representing a number of hours. One or more ASCII digits followed by a U+004D LATIN CAPITAL LETTER M
character, representing a number of minutes. The following components: One or more ASCII digits, representing a number of seconds. Optionally, a U+002E FULL STOP character (.) followed by one, two, or three
ASCII digits, representing a fraction of a second. A U+0053 LATIN CAPITAL LETTER S character. This, as with a number of other date- and time-related microsyntaxes defined in
this specification, is based on one of the formats defined in ISO 8601. [ISO8601] One or more duration time components, each with
a different duration time component scale, in any order; the sum of the represented
seconds being equal to the number of seconds in t. A duration time component is a string consisting of the following components: Zero or more ASCII whitespace. One or more ASCII digits, representing a number of time units, scaled by
the duration time component scale specified (see below) to represent a number of
seconds. If the duration time component scale specified is 1 (i.e. the units are
seconds), then, optionally, a U+002E FULL STOP character (.) followed by one, two, or three
ASCII digits, representing a fraction of a second. Zero or more ASCII whitespace. One of the following characters, representing the duration time component scale
of the time unit used in the numeric part of the duration time component: Zero or more ASCII whitespace. This is not based on any of the formats in ISO 8601. It is intended to be a more
human-readable alternative to the ISO 8601 duration format. The rules to parse a duration string are as follows. This will return either a duration or nothing. If at any point the algorithm says that it
"fails", this means that it is aborted at that point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Let months, seconds, and component
count all be zero. Let M-disambiguator be minutes. This flag's other value is months. It is used to disambiguate the "M"
unit in ISO8601 durations, which use the same unit for months and minutes. Months are not
allowed, but are parsed for future compatibility and to avoid misinterpreting ISO8601 durations
that would be valid in other contexts. Skip ASCII whitespace within input given
position. If position is past the end of input, then
fail. If the character in input pointed to by position is a U+0050 LATIN
CAPITAL LETTER P character, then advance position to the next character, set
M-disambiguator to months, and skip ASCII whitespace within
input given position. While true: Let units be undefined. It will be assigned one of the following
values: years, months, weeks, days, hours, minutes,
and seconds. Let next character be undefined. It is used to process characters
from the input. If position is past the end of input, then break. If the character in input pointed to by position is a U+0054 LATIN
CAPITAL LETTER T character, then advance position to the next character, set
M-disambiguator to minutes, skip ASCII whitespace within
input given position, and continue. Set next character to the character in input
pointed to by position. If next character is a U+002E FULL STOP character (.), then let N equal zero. (Do not advance position. That is taken care
of below.) Otherwise, if next character is an ASCII
digit, then collect a sequence of code points that are ASCII
digits from input given position, interpret the resulting
sequence as a base-ten integer, and let N be that number. Otherwise next character is not part of a number; fail. If position is past the end of input, then
fail. Set next character to the character in input
pointed to by position, and this time advance position
to the next character. (If next character was a U+002E FULL STOP character
(.) before, it will still be that character this time.) If next character is U+002E (.), then: Collect a sequence of code points that are ASCII digits from
input given position. Let s be the resulting
sequence. If s is the empty string, then fail. Let length be the number of characters in s. Let fraction be the result of interpreting s
as a base-ten integer, and then dividing that number by 10length. Increment N by fraction. Skip ASCII whitespace within input given
position. If position is past the end of input, then
fail. Set next character to the character in input
pointed to by position, and advance position to the
next character. If next character is neither a U+0053 LATIN CAPITAL LETTER S
character nor a U+0073 LATIN SMALL LETTER S character, then fail. Set units to seconds. Otherwise: If next character is ASCII whitespace, then skip ASCII
whitespace within input given position, set next
character to the character in input pointed to by position, and
advance position to the next character. If next character is a U+0059 LATIN CAPITAL LETTER Y character, or a
U+0079 LATIN SMALL LETTER Y character, set units to years and set
M-disambiguator to months. If next character is a U+004D LATIN CAPITAL LETTER M character or a
U+006D LATIN SMALL LETTER M character, and M-disambiguator is
months, then set units to months. If next character is a U+0057 LATIN CAPITAL LETTER W character or a
U+0077 LATIN SMALL LETTER W character, set units to weeks and set
M-disambiguator to minutes. If next character is a U+0044 LATIN CAPITAL LETTER D character or a
U+0064 LATIN SMALL LETTER D character, set units to days and set
M-disambiguator to minutes. If next character is a U+0048 LATIN CAPITAL LETTER H character or a
U+0068 LATIN SMALL LETTER H character, set units to hours and set
M-disambiguator to minutes. If next character is a U+004D LATIN CAPITAL LETTER M character or a
U+006D LATIN SMALL LETTER M character, and M-disambiguator is
minutes, then set units to minutes. If next character is a U+0053 LATIN CAPITAL LETTER S character or a
U+0073 LATIN SMALL LETTER S character, set units to seconds and
set M-disambiguator to minutes. Otherwise if next character is none of the above characters, then
fail. Increment component count. Let multiplier be 1. If units is years, multiply multiplier by
12 and set units to months. If units is months, add the product of N and
multiplier to months. Otherwise: If units is weeks, multiply multiplier
by 7 and set units to days. If units is days, multiply multiplier
by 24 and set units to hours. If units is hours, multiply multiplier
by 60 and set units to minutes. If units is minutes, multiply multiplier by 60 and set units to seconds. Forcibly, units is now seconds. Add the product of N and multiplier to seconds. Skip ASCII whitespace within input given
position. If component count is zero,
fail. If months is not zero, fail. Return the duration consisting of seconds seconds. A string is a valid date string with optional time if
it is also one of the following: The rules to parse a date or time string are as follows. The algorithm will return
either a date, a time, a global date and time, or nothing. If at any point the algorithm
says that it "fails", this means that it is aborted at that point and returns nothing. Let input be the string being parsed. Let position be a pointer into input, initially
pointing at the start of the string. Set start position to the same position as position. Set the date present and time present flags to
true. Parse a date component to obtain year, month, and day. If this fails, then set the date
present flag to false. If date present is true, and position is not beyond
the end of input, and the character at position is
either a U+0054 LATIN CAPITAL LETTER T character (T) or a U+0020 SPACE character, then advance
position to the next character in input. Otherwise, if date present is true, and either position is beyond the end of input or the character at position is neither a U+0054 LATIN CAPITAL LETTER T character (T) nor a U+0020
SPACE character, then set time present to false. Otherwise, if date present is false, set position
back to the same position as start position. If the time present flag is true, then parse a time
component to obtain hour, minute, and second. If this returns nothing, then fail. If the date present and time present flags are
both true, but position is beyond the end of input, then
fail. If the date present and time present flags are
both true, parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this
returns nothing, then fail. If position is not beyond the end of input, then fail. If the date present flag is true and the time present
flag is false, then let date be the date with year year,
month month, and day day, and return date. Otherwise, if the time present flag is true and the date
present flag is false, then let time be the time with hour hour, minute minute, and second second,
and return time. Otherwise, let time be the moment in time at year year, month month, day day, hours hour, minute minute, second second,
subtracting timezonehours hours and timezoneminutes minutes, that moment in time being a moment
in the UTC time zone; let timezone be timezonehours hours and timezoneminutes
minutes from UTC; and return time and timezone. A simple color consists of three 8-bit numbers in the
range 0 to 255, inclusive, representing the red, green, and blue components of the color
respectively, in the 'srgb' color space. A string is a valid simple color if it is
exactly seven characters long, and the first character is a U+0023 NUMBER SIGN character (#), and
the remaining six characters are all ASCII hex digits, with the first two digits
representing the red component, the middle two digits representing the green component, and the
last two digits representing the blue component, in hexadecimal. A string is a valid lowercase simple
color if it is a valid simple color and doesn't use any characters in the range
U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F. The rules for parsing simple color
values are as given in the following algorithm. When invoked, the steps must be followed in
the order given, aborting at the first step that returns a value. This algorithm will return
either a simple color or an error. Let input be the string being parsed. If input is not exactly seven characters long, then return an
error. If the first character in input is not a U+0023 NUMBER SIGN character
(#), then return an error. If the last six characters of input are not all ASCII hex
digits, then return an error. Let result be a simple color. Interpret the second and third characters as a hexadecimal number and let the result be
the red component of result. Interpret the fourth and fifth characters as a hexadecimal number and let the result be
the green component of result. Interpret the sixth and seventh characters as a hexadecimal number and let the result be
the blue component of result. Return result. The rules for serializing
simple color values given a simple color are as given in the following
algorithm: Let result be a string consisting of a single U+0023 NUMBER SIGN
character (#). Convert the red, green, and blue components in turn to two-digit hexadecimal numbers using
ASCII lower hex digits, zero-padding if necessary,
and append these numbers to result, in the order red, green, blue. Return result, which will be a valid lowercase simple
color. Some obsolete legacy attributes parse colors in a more complicated manner, using the
rules for parsing a legacy color
value, which are given in the following algorithm. When invoked, the steps must be followed
in the order given, aborting at the first step that returns a value. This algorithm will return
either a simple color or an error. Let input be the string being parsed. If input is the empty string, then return an error. Strip leading and trailing ASCII whitespace from input. If input is an ASCII case-insensitive match for the
string " If input is an ASCII case-insensitive match for one of the
named colors, then return the simple color
corresponding to that keyword. [CSSCOLOR] CSS2 System
Colors are not recognized. If input's code point length is four, and the first character in
input is U+0023 (#), and the last three characters of input are all
ASCII hex digits, then: Let result be a simple color. Interpret the second character of input as a hexadecimal digit; let
the red component of result be the resulting number multiplied by 17. Interpret the third character of input as a hexadecimal digit; let
the green component of result be the resulting number multiplied by 17. Interpret the fourth character of input as a hexadecimal digit; let
the blue component of result be the resulting number multiplied by 17. Return result. Replace any code points greater than U+FFFF in
input (i.e., any characters that are not in the basic multilingual plane) with the
two-character string " If input's code point length is greater than 128, truncate
input, leaving only the first 128 characters. If the first character in input is a U+0023 NUMBER SIGN character (#),
remove it. Replace any character in input that is not an ASCII hex digit with the character U+0030 DIGIT ZERO (0). While input's code point length is zero or not a multiple of
three, append a U+0030 DIGIT ZERO (0) character to input. Split input into three strings of equal code point length, to
obtain three components. Let length be the code point length that all of
those components have (one third the code point length of input). If length is greater than 8, then remove the leading length-8 characters in each component, and let length be 8. While length is greater than two and the first character in each
component is a U+0030 DIGIT ZERO (0) character, remove that character and reduce length by one. If length is still greater than two, truncate each component,
leaving only the first two characters in each. Let result be a simple color. Interpret the first component as a hexadecimal number; let the red component of result be the resulting number. Interpret the second component as a hexadecimal number; let the green component of result be the resulting number. Interpret the third component as a hexadecimal number; let the blue component of result be the resulting number. Return result. The 2D graphics context has a separate
color syntax that also handles opacity. A set of space-separated tokens is a string containing zero or more words
(known as tokens) separated by one or more ASCII whitespace, where words consist of
any string of one or more characters, none of which are ASCII whitespace. A string containing a set of space-separated tokens may have leading or trailing
ASCII whitespace. An unordered set of unique space-separated tokens is a set of
space-separated tokens where none of the tokens are duplicated. An ordered set of unique space-separated tokens is a set of space-separated
tokens where none of the tokens are duplicated but where the order of the tokens is
meaningful. Sets of space-separated tokens sometimes
have a defined set of allowed values. When a set of allowed values is defined, the tokens must all
be from that list of allowed values; other values are non-conforming. If no such set of allowed
values is provided, then all values are conforming. How tokens in a set of space-separated tokens are to be compared
(e.g. case-sensitively or not) is defined on a per-set basis. A set of comma-separated tokens is a string containing zero or more tokens each
separated from the next by a single U+002C COMMA character (,), where tokens consist of any string
of zero or more characters, neither beginning nor ending with ASCII whitespace, nor
containing any U+002C COMMA characters (,), and optionally surrounded by ASCII
whitespace. For instance, the string " Sets of comma-separated tokens sometimes
have further restrictions on what consists a valid token. When such restrictions are defined, the
tokens must all fit within those restrictions; other values are non-conforming. If no such
restrictions are specified, then all values are conforming. A valid hash-name reference to an element of type type is a
string consisting of a U+0023 NUMBER SIGN character (#) followed by a string which exactly matches
the value of the The rules for parsing a hash-name reference to an element of type type,
given a context node scope, are as follows: If the string being parsed does not contain a U+0023 NUMBER SIGN character, or if the
first such character in the string is the last character in the string, then return null. Let s be the string from the character immediately after the first
U+0023 NUMBER SIGN character in the string being parsed up to the end of that string. Return the first element of type type in scope's tree, in
tree order, that has an Although A string is a valid media query list if it matches the A string matches the environment of the user if it is the empty string, a string
consisting of only ASCII whitespace, or is a media query list that matches the user's
environment according to the definitions given in Media Queries. [MQ] A unique internal value is a value that is serializable, comparable by value, and
never exposed to script. To create a new unique internal value, return a unique internal value
that has never previously been returned by this algorithm. A string is a valid non-empty URL if it is a valid URL string but it is
not the empty string. A string is a valid URL potentially surrounded by spaces if, after stripping leading and trailing ASCII
whitespace from it, it is a valid URL string. A string is a valid non-empty URL potentially surrounded by spaces if, after stripping leading and trailing ASCII
whitespace from it, it is a valid non-empty URL. This specification defines the URL This specification defines the URL This specification defines the URL The fallback base URL of a If document is an Assert: document's about base URL is non-null. Return document's about base
URL. If document's URL matches
Return document's URL. The document base URL of a If there is no Otherwise, return the frozen base URL of the first A URL matches Such a URL's query and fragment can be non-null. For example, the URL
record created by parsing " A URL matches The reason that matches Parsing a URL is the process of taking a string and obtaining the URL record that
it represents. While this process is defined in URL, the HTML standard defines
several wrappers to abstract base URLs and encodings. [URL] Most new APIs are to use parse a URL. Older APIs and HTML elements
might have reason to use encoding-parse a URL. When a
custom base URL is needed or no base URL is desired, the URL parser can of course be
used directly as well. To parse a URL, given a string url, relative to a
Let baseURL be environment's base
URL, if environment is a Return the result of applying the URL parser to url, with
baseURL. To encoding-parse a URL,
given a string url, relative to a Let encoding be UTF-8. If environment is a Otherwise, if environment's relevant global object is a
Let baseURL be environment's base
URL, if environment is a Return the result of applying the URL parser to url, with
baseURL and encoding. To encoding-parse-and-serialize a
URL, given a string url, relative to a Let url be the result of encoding-parsing a URL given
url, relative to environment. If url is failure, then return failure. Return the result of applying the URL
serializer to url. When a document's document base URL changes, all elements in that document are
affected by a base URL change. The following are base URL change steps, which run when an element is
affected by a base URL change (as defined by DOM): If the URL identified by the hyperlink is being shown to the user, or if any
data derived from that URL is affecting the display, then the For example, the CSS If the hyperlink has a If the URL identified by the The element is not directly affected. For instance, changing the base URL doesn't affect the image displayed by
A response whose type is " A response whose type is " A response's unsafe response is its
internal response if it has one, and the response itself otherwise. To create a potential-CORS request, given a url, destination,
corsAttributeState, and an optional same-origin fallback flag, run these
steps: Let mode be " If same-origin fallback flag is set and mode is " Let credentialsMode be " If corsAttributeState is Anonymous, set credentialsMode to " Let request be a new request whose
URL is url, destination is destination,
mode is mode, credentials mode is
credentialsMode, and whose use-URL-credentials flag is set. The Content-Type metadata of a resource must be obtained and
interpreted in a manner consistent with the requirements of MIME Sniffing.
[MIMESNIFF] The of a
resource must be found in a manner consistent with the requirements given in MIME
Sniffing. [MIMESNIFF] The , the , and the are also defined in MIME Sniffing.
These rules return a MIME type as their result. [MIMESNIFF] It is imperative that the rules in MIME Sniffing be
followed exactly. When a user agent uses different heuristics for content type detection than the
server expects, security problems can occur. For more details, see MIME Sniffing.
[MIMESNIFF] The algorithm for extracting a character encoding from a Let position be a pointer into s, initially
pointing at the start of the string. Loop: Find the first seven characters in s after position that are an ASCII case-insensitive match for the word " Skip any ASCII whitespace that immediately follow the word " If the next character is not a U+003D EQUALS SIGN (=), then move position to point just before that next character, and jump back to the step
labeled loop. Skip any ASCII whitespace that immediately follow the equals sign (there
might not be any). Process the next character as follows: This algorithm is distinct from those in the HTTP specifications (for example,
HTTP doesn't allow the use of single quotes and requires supporting a backslash-escape mechanism
that is not supported by this algorithm). While the
algorithm is used in contexts that, historically, were related to HTTP, the syntax as supported by
implementations diverged some time ago. [HTTP] Support in all current engines. A CORS settings attribute is an enumerated attribute. The following
table lists the keywords and states for the attribute — the states given in the first cell
of the rows with keywords give the states to which those keywords map. The attribute's invalid value default is the Anonymous state, and its missing value default is the No CORS state.
For the purposes of reflection, the canonical keyword
for the Anonymous state is the The majority of fetches governed by CORS settings
attributes will be done via the create a potential-CORS request algorithm. For more modern features, where the request's mode is always " A referrer policy attribute is an enumerated attribute. Each
referrer policy, including the empty string, is a keyword for this attribute, mapping
to a state of the same name. The attribute's invalid value default and missing value default are both the empty string state. The impact of these states on the processing model of various fetches is defined in more detail throughout this specification, in
Fetch, and in Referrer Policy. [FETCH]
[REFERRERPOLICY] Several signals can contribute to which processing model is used for a given fetch; a referrer policy attribute is only one of
them. In general, the order in which these signals are processed are: First, the presence of a Then, the value of a referrer policy attribute; Then, the presence of any Finally, the ` Support in all current engines. A Elements that have a Returns the value set for element's cryptographic nonce. If the setter was not
used, this will be the value originally found in the Updates element's cryptographic nonce value. The Note how the setter for the The following attribute change
steps are used for the If element does not include If localName is not If value is null, then set element's
[[CryptographicNonce]] to the empty string. Otherwise, set element's [[CryptographicNonce]] to
value. Whenever an element including Let CSP list be element's shadow-including root's policy
container's CSP list. If CSP list contains a header-delivered Content Security Policy, and
element has a Let nonce be element's
[[CryptographicNonce]]. Set an attribute value for
element using " Set element's [[CryptographicNonce]] to
nonce. If element's [[CryptographicNonce]] were not restored it
would be the empty string at this point. The cloning steps for elements that
include Support in all current engines. A lazy loading attribute is an enumerated attribute. The following
table lists the keywords and states for the attribute — the keywords in the left column map
to the states in the cell in the second column on the same row as the keyword. The attribute directs the user agent to fetch a resource immediately or to defer fetching until
some conditions associated with the element are met, according to the attribute's current
state. The attribute's missing value default and invalid value default are both the Eager state. The will lazy load element steps, given an element element,
are as follows: If scripting is disabled for element,
then return false. This is an anti-tracking measure, because if a user agent supported lazy loading
when scripting is disabled, it would still be possible for a site to track a user's approximate
scroll position throughout a session, by strategically placing images in a page's markup such
that a server can track how many images are requested and when. If element's lazy loading attribute is in the Lazy state, then return true. Return false. Each For Each To start intersection-observing a lazy loading element element, run these
steps: Let doc be element's node document. If doc's lazy load intersection observer is null, set it to a new
The intention is to use the original value of the
The callback is these steps, with arguments entries and
observer: For each entry in entries using a method of
iteration which does not trigger developer-modifiable array accessors or
iteration hooks: Let resumptionSteps be null. If entry. If resumptionSteps is null, then return. Stop intersection-observing a lazy loading element for
entry. Set entry. Invoke resumptionSteps. The intention is to use the original value of the
The options is an This allows for fetching the image during scrolling, when it does not yet —
but is about to — intersect the viewport. The lazy load root margin suggestions imply dynamic changes to the
value, but the Call doc's lazy load intersection observer's The intention is to use the original value of the To stop intersection-observing a lazy loading element element, run these
steps: Let doc be element's node document. Assert: doc's lazy load intersection observer is not
null. Call doc's lazy load intersection observer The intention is to use the original value of the
Set a minimum value that most often results in the resources being loaded before they
intersect the viewport under normal usage patterns for the given device. The typical scrolling speed: increase the value for devices with faster typical scrolling
speeds. The current scrolling speed or momentum: the UA can attempt to predict where the scrolling
will likely stop, and adjust the value accordingly. The network quality: increase the value for slow or high-latency connections. User preferences can influence the value. It is important for privacy that the
lazy load root margin not leak additional information. For example, the typical
scrolling speed on the current device could be imprecise so as to not introduce a new
fingerprinting vector. A blocking attribute explicitly indicates that certain operations should be blocked
on the fetching of an external resource. The operations that can be blocked are represented by
possible blocking tokens, which are strings listed by
the following table: In the future, there might be more possible blocking tokens. A blocking attribute must have a value that is an unordered set of unique
space-separated tokens, each of which are possible
blocking tokens. The supported tokens of a
blocking attribute are the possible blocking
tokens. Any element can have at most one blocking attribute. The blocking tokens set for an element el are the result of the
following steps: Let value be the value of el's blocking attribute, or
the empty string if no such attribute exists. Set value to value, converted to ASCII lowercase. Let rawTokens be the result of splitting value on ASCII whitespace. Return a set containing the elements of rawTokens that are possible blocking tokens. An element is potentially render-blocking if its blocking tokens set
contains " A fetch priority attribute is an enumerated attribute. The following
table lists the keywords and states for the attribute — the keywords in the left column map to
the states in the cell in the second column on the same row as the keyword. The attribute's missing value default and
invalid value default are both the
auto state. The building blocks for reflecting are as follows: A reflected target is an element or A reflected IDL attribute is an attribute interface member. A reflected content attribute name is a string. When the reflected
target is an element, it represents the local name of a content attribute whose namespace
is null. When the reflected target is an A reflected IDL attribute can be defined to reflect a
reflected content attribute name of a reflected target. In general this
means that the IDL attribute getter returns the current value of the content attribute, and the
setter changes the value of the content attribute to the given value. If the reflected target is an element, then the reflected IDL
attribute can additionally declare to support The Reflected targets have these associated algorithms:
For a reflected target that is an element element, these are defined as
follows: Return element. Let attribute be the result of running get an attribute by namespace and local
name given null, the reflected content attribute name, and
element. If attribute is null, then return null. Return attribute's value. Set an attribute value
given element, the reflected content attribute name, and
value. Remove an attribute
by namespace and local name given null, the reflected content attribute name,
and element. For a reflected target that is an Return elementInternals's target
element. If elementInternals's target element's
internal content attribute map[the reflected content attribute name]
does not exist, then return null.
Return elementInternals's target
element's internal content attribute map[the reflected content
attribute name]. Set elementInternals's target element's internal content attribute map[the
reflected content attribute name] to value. Remove elementInternals's target element's internal content attribute map[the
reflected content attribute name]. This results in somewhat redundant data structures for
IDL attributes of type If a reflected IDL attribute has the type The getter steps are: Let element be the result of running this's get the
element. Let contentAttributeValue be the result of running this's
get the content attribute. Let attributeDefinition be the attribute definition of element's
content attribute whose namespace is null and local name is the reflected content
attribute name.
If attributeDefinition indicates it is an enumerated attribute and
the reflected IDL attribute is defined to be limited to only known
values: If contentAttributeValue does not correspond to any state of
attributeDefinition (e.g., it is null and there is no missing value default), or that it is in a state of
attributeDefinition with no associated keyword value, then return the empty
string. Return the canonical keyword for the state of
attributeDefinition that contentAttributeValue corresponds to. If contentAttributeValue is null, then return the empty string. Return contentAttributeValue. The setter steps are to run this's set the content attribute
with the given value.
If a reflected IDL attribute has the type The getter steps are: Let element be the result of running this's get the
element. Let contentAttributeValue be the result of running this's
get the content attribute. Let attributeDefinition be the attribute definition of element's
content attribute whose namespace is null and local name is the reflected content
attribute name.
Assert: attributeDefinition indicates it is an enumerated
attribute. Assert: the reflected IDL attribute is limited to only
known values. Assert: contentAttributeValue corresponds to a state of
attributeDefinition. If contentAttributeValue corresponds to a state of
attributeDefinition with no associated keyword value, then return null. Return the canonical keyword for the state of
attributeDefinition that contentAttributeValue corresponds to. The setter steps are: If the given value is null, then run this's delete the content
attribute. Otherwise, run this's set the content attribute with the given
value. If a reflected IDL attribute has the type The getter steps are: Let element be the result of running this's get the
element. Let contentAttributeValue be the result of running this's
get the content attribute. Let attributeDefinition be the attribute definition of element's
content attribute whose namespace is null and local name is the reflected content
attribute name.
If attributeDefinition indicates it contains a URL: If contentAttributeValue is null, then return the empty string. Let urlString be the result of encoding-parsing-and-serializing a
URL given contentAttributeValue, relative to element's
node document. If urlString is not failure, then return urlString. Return contentAttributeValue, converted to a scalar value string. The setter steps are to run this's set the content attribute
with the given value.
If a reflected IDL attribute has the type The getter steps are: Let contentAttributeValue be the result of running this's
get the content attribute. If contentAttributeValue is null, then return false. Return true. The setter steps are: If the given value is false, then run this's delete the content
attribute. If the given value is true, then run this's set the content
attribute with the empty string.
This corresponds to the rules for boolean content
attributes.
If a reflected IDL attribute has the type The getter steps are: Let contentAttributeValue be the result of running this's
get the content attribute. If contentAttributeValue is not null:
Let parsedValue be the result of integer parsing contentAttributeValue if the reflected IDL
attribute is not limited to only non-negative numbers; otherwise the
result of non-negative integer
parsing contentAttributeValue. If parsedValue is not an error and is within the If the reflected IDL attribute has a default value, then
return defaultValue. If the reflected IDL attribute is limited to only non-negative
numbers, then return −1. Return 0. The setter steps are: If the reflected IDL attribute is limited to only non-negative
numbers and the given value is negative, then throw an
" If a reflected IDL attribute has the type The getter steps are: Let contentAttributeValue be the result of running this's
get the content attribute. Let minimum be 0. If the reflected IDL attribute is limited to only positive
numbers or limited to only positive numbers with fallback, then set
minimum to 1. If the reflected IDL attribute is clamped to the range, then
set minimum to clampedMin. Let maximum be 2147483647 if the reflected IDL attribute is not
clamped to the range; otherwise clampedMax. If contentAttributeValue is not null:
Let parsedValue be the result of non-negative integer parsing
contentAttributeValue. If parsedValue is not an error and is in the range minimum to
maximum, inclusive, then return parsedValue. If parsedValue is not an error and the reflected IDL attribute is
clamped to the range:
If parsedValue is less than minimum, then return
minimum. Return maximum. If the reflected IDL attribute has a default value, then
return defaultValue. Return minimum. The setter steps are: If the reflected IDL attribute is limited to only positive
numbers and the given value is 0, then throw an
" If the reflected IDL attribute is limited to only positive
numbers or limited to only positive numbers with fallback, then set
minimum to 1. Let newValue be minimum. If the reflected IDL attribute has a default value, then set
newValue to defaultValue. If the given value is in the range minimum to 2147483647, inclusive,
then set newValue to it. Run this's set the content attribute with newValue
converted to the shortest possible string representing the number as a valid non-negative
integer. Clamped to the range has no effect on the setter steps. If a reflected IDL attribute has the type The getter steps are: Let contentAttributeValue be the result of running this's
get the content attribute. If contentAttributeValue is not null:
Let parsedValue be the result of floating-point number parsing
contentAttributeValue. If parsedValue is not an error and is greater than 0, then return
parsedValue. If parsedValue is not an error and the reflected IDL attribute
is not limited to only positive numbers, then return
parsedValue. If the reflected IDL attribute has a default value, then
return defaultValue. Return 0. The setter steps are: If the reflected IDL attribute is limited to only positive
numbers and the given value is not greater than 0, then return. Run this's set the content attribute with the given value,
converted to the best representation of the number as a floating-point
number. The values Infinity and Not-a-Number (NaN) values throw an exception on setting,
as defined in Web IDL. [WEBIDL] If a reflected IDL attribute has the type If a reflected IDL attribute has the type Let attr be the reflected content attribute name. Its reflected target has an explicitly set
attr-element, which is a weak reference to an element or null. It is initially
null. Its reflected target has an attr-associated element. To compute the attr-associated
element for such a reflected target reflectedTarget: Let element be the result of running reflectedTarget's get
the element. Let contentAttributeValue be the result of running
reflectedTarget's get the content attribute. If reflectedTarget's explicitly set attr-element is not
null: If reflectedTarget's explicitly set attr-element is
a descendant of any of element's shadow-including ancestors, then return
reflectedTarget's explicitly set attr-element. Return null. Otherwise, if contentAttributeValue is not null, return the first element
candidate, in tree order, that meets the following criteria: If no such element exists, then return null. Return null. The getter steps are to return this's attr-associated
element. The setter steps are: If the given value is null, then: Set this's explicitly set attr-element to
null. Run this's delete the content attribute. Return. Run this's set the content attribute with the empty
string. Set this's explicitly set attr-element to a weak
reference to the given value. For element reflected targets only: the following
attribute change steps, given
element, localName, oldValue, value, and
namespace, are used to synchronize between the content attribute and the IDL
attribute: If localName is not attr or namespace is not null, then
return. Set element's explicitly set attr-element to
null. Reflected IDL attributes of this
type are strongly encouraged to have their identifier end in " If a reflected IDL attribute has the type Let attr be the reflected content attribute name. Its reflected target has an explicitly set
attr-elements, which is either a list of weak references to
elements or null. It is initially null. Its reflected target has a cached attr-associated
elements, which is a Its reflected target has an attr-associated elements. To compute the
attr-associated elements for such a reflected target
reflectedTarget: Let elements be an empty list. Let element be the result of running reflectedTarget's get
the element. If reflectedTarget's explicitly set attr-elements is not
null: For each attrElement in
reflectedTarget's explicitly set attr-elements: If attrElement is not a descendant of any of
element's shadow-including
ancestors, then continue. Append attrElement to
elements. Otherwise: Let contentAttributeValue be the result of running
reflectedTarget's get the content attribute.
If contentAttributeValue is null, then return null. Let tokens be contentAttributeValue, split on ASCII whitespace.
For each id of tokens: Let candidate be the first element, in tree order, that meets
the following criteria: If no such element exists, then continue. Append candidate to
elements. Return elements. The getter steps are: Let elements be this's attr-associated
elements. If the contents of elements is equal to the contents of this's
cached attr-associated elements, then return this's
cached attr-associated elements. Let elementsAsFrozenArray be elements, converted to a Set this's cached attr-associated elements to
elementsAsFrozenArray. Return elementsAsFrozenArray. This extra caching layer is necessary to preserve the invariant that The setter steps are: If the given value is null: Set this's explicitly set attr-elements to
null. Run this's delete the content attribute. Return. Run this's set the content attribute with the empty
string. Let elements be an empty list. For each element in the given value: Append a weak reference to element to
elements. Set this's explicitly set attr-elements to
elements. For element reflected targets only: the following
attribute change steps, given
element, localName, oldValue, value, and
namespace, are used to synchronize between the content attribute and the IDL
attribute: If localName is not attr or namespace is not null, then
return. Set element's explicitly set attr-elements to
null. Reflected IDL attributes of this
type are strongly encouraged to have their identifier end in " Reflection is primarily about improving web developer ergonomics
by giving them typed access to content attributes through reflected IDL attributes. The ultimate source of truth, which the web platform
builds upon, is the content attributes themselves. That is, specification authors must not use the
reflected IDL attribute getter or setter steps, but instead must use the content
attribute presence and value. (Or an abstraction on top, such as the state of an enumerated
attribute.) Two important exceptions to this are reflected IDL
attributes whose type is one of the following: A reflected target's explicitly set attr-element,
explicitly set attr-elements, and cached attr-associated
elements are to be treated as internal implementation details and not to be built upon. The The All Objects that implement the Objects that implement the The ToBoolean abstract operation in JavaScript returns
false when given objects implementing the The IsLooselyEqual abstract operation,
when given objects implementing the The These special behaviors are motivated by a desire for compatibility with two classes of legacy
content: one that uses the presence of The object's supported property indices are as defined for
The supported property names consist of the non-empty values of all the The The indexed property getter must return the result of getting the "all"-indexed element from this
given the passed index. The The If nameOrIndex was not provided, return null. Return the result of getting the
"all"-indexed or named element(s) from this, given
nameOrIndex. The following elements are "all"-named elements:
To get the "all"-indexed element from an
To get the "all"-named element(s) from an
If name is the empty string, return null. Let subCollection be an "all"-named elements with a elements with an ID equal to name. If there is exactly one element in subCollection, then return that
element. Otherwise, if subCollection is empty, return null. Otherwise, return subCollection. To get the "all"-indexed or named
element(s) from an If nameOrIndex, converted to a
JavaScript String value, is an array index property name, return the result of getting the "all"-indexed element from
collection given the number represented by nameOrIndex. Return the result of getting the "all"-named
element(s) from collection given nameOrIndex. If argumentsList's size is zero, or if
argumentsList[0] is undefined, return null. Let nameOrIndex be the result of converting argumentsList[0] to a Return the result of converting
result to an ECMAScript value. The thisArgument is ignored, and thus code such as The Support in all current engines. Support in all current engines. Returns the number of elements in collection. Returns the item at index index in collection. The items are sorted
in tree order. HTMLFormControlsCollection/namedItem Support in all current engines. Returns the item with ID or If there are multiple matching items, then a Returns the value of the first checked radio button represented by
radioNodeList. Checks the first first radio button represented by radioNodeList that has value
value. The object's supported property indices are as defined for
The supported property names consist of the non-empty values of all the The Members of the Support in all current engines. The Let element be the first element in tree order
represented by the If element is null, return the empty string. If element is an element with no Otherwise, return the value of element's On setting, the If the new value is the string " Otherwise: let element be the first element in tree order
represented by the If element is not null, then set its checkedness to true. Support in all current engines. The Returns the number of elements in collection. When set to a smaller number than the existing length, truncates the number of
When set to a greater number than the existing length, if that number is less than or equal
to 100000, adds new blank Returns the item at index index in collection. The items are sorted
in tree order. When index is a greater number than the number of items in collection,
adds new blank When set to null, removes the item at index index from collection. When set to an Returns the item with ID or If there are multiple matching items, then the first is returned. Inserts element before the node given by before. The before argument can be a number, in which case element is inserted
before the item with that number, or an element from collection, in which case
element is inserted before that element. If before is omitted, null, or a number out of range, then element will
be added at the end of the list. Throws a " Removes the item with index index from collection. Returns the index of the first selected item, if any, or −1 if there is no selected
item. Changes the selection to the The object's supported property indices are as defined for
The The Let current be the number of nodes represented by the
collection. If the given value is greater than current, then: If the given value is less than current, then: Let n be current − value. Remove the last n nodes in the collection from their parent nodes. Setting The supported property names consist of the non-empty values of all the When the user agent is to set the value of a new
indexed property or set the value of an existing indexed property for a given
property index index to a new value value, it must run the following
algorithm: If value is null, invoke the steps for the Let length be the number of nodes represented by the
collection. Let n be index minus length. If n is greater than zero, then append a If n is greater than or equal to zero, append value to the The If element is an ancestor of the If before is a node, then let reference be that
node. Otherwise, if before is an integer, and there is a beforeth node in the collection, let reference be that node.
Otherwise, let reference be null. If reference is not null, let parent be the parent
node of reference. Otherwise, let parent be the
Pre-insert element into parent node before
reference. The If the number of nodes represented by the collection is zero, return. If index is not a number greater than or equal to 0 and less than the
number of nodes represented by the collection, return. Let element be the indexth element in the
collection. Remove element from its parent node. The Support in all current engines. The New APIs must use Returns the number of strings in strings. Returns the string with index index from strings. Returns true if strings contains string, and false
otherwise. Each The Support in all current engines. The Support in all current engines. The Support in all current engines. The To support passing JavaScript objects,
including platform objects, across realm
boundaries, this specification defines the following infrastructure for
serializing and deserializing objects, including in some cases transferring the underlying data
instead of copying it. Collectively this serialization/deserialization process is known as
"structured cloning", although most APIs perform separate serialization and deserialization steps.
(With the notable exception being the This section uses the terminology and typographic conventions from the JavaScript
specification. [JAVASCRIPT] /developer.mozilla.org/en-US/docs/Glossary/Serializable_object /developer.mozilla.org/en-US/docs/Glossary/Serializable_object /developer.mozilla.org/en-US/docs/Glossary/Serializable_object /developer.mozilla.org/en-US/docs/Glossary/Serializable_object /developer.mozilla.org/en-US/docs/Glossary/Serializable_object /developer.mozilla.org/en-US/docs/Glossary/Serializable_object /developer.mozilla.org/en-US/docs/Glossary/Serializable_object Serializable objects support being serialized, and later deserialized, in a way
that is independent of any given realm. This allows them to be stored on disk and
later restored, or cloned across agent and even agent cluster
boundaries. Not all objects are serializable objects, and not all aspects of objects that are
serializable objects are necessarily preserved when they are serialized. Platform objects can be serializable objects
if their primary interface is decorated with the A set of steps that serializes the data in value into fields of
serialized. The resulting data serialized into serialized must be
independent of any realm. These steps may throw an exception if serialization is not possible. These steps may perform a sub-serialization to serialize nested data
structures. They should not call StructuredSerialize directly, as doing so will
omit the important memory argument. The introduction of these steps should omit mention of the forStorage argument if
it is not relevant to the algorithm. A set of steps that deserializes the data in serialized, using it to set up
value as appropriate. value will be a newly-created instance of the
platform object type in question, with none of its internal data set up; setting
that up is the job of these steps. These steps may throw an exception if deserialization is not possible. These steps may perform a sub-deserialization to deserialize nested data
structures. They should not call StructuredDeserialize directly, as doing so will
omit the important targetRealm and memory arguments. It is up to the definition of individual platform objects to determine what data is serialized
and deserialized by these steps. Typically the steps are very symmetric. The For a given platform object, only the object's primary interface is
considered during the (de)serialization process. Thus, if inheritance is involved in defining the
interface, each Let's say we were defining a platform object We could then define Set serialized.[[Name]] to value's associated name
value. Let serializedBestFriend be the sub-serialization of
value's associated best friend value. Set serialized.[[BestFriend]] to serializedBestFriend. Set value's associated name value to
serialized.[[Name]]. Let deserializedBestFriend be the sub-deserialization of
serialized.[[BestFriend]]. Set value's associated best friend value to
deserializedBestFriend. Objects defined in the JavaScript specification are handled by the
StructuredSerialize abstract operation directly. Originally, this specification defined the concept of
"cloneable objects", which could be cloned from one realm to another. However, to
better specify the behavior of certain more complex situations, the model was updated to make the
serialization and deserialization explicit. Transferable objects support being transferred across agents. Transferring is effectively recreating the object while sharing a
reference to the underlying data and then detaching the object being transferred. This is useful
to transfer ownership of expensive resources. Not all objects are transferable
objects and not all aspects of objects that are transferable objects are
necessarily preserved when transferred. Transferring is an irreversible and non-idempotent operation. Once an object has
been transferred, it cannot be transferred, or indeed used, again. Platform objects can be transferable objects
if their primary interface is decorated with the A set of steps that transfers the data in value into fields of
dataHolder. The resulting data held in dataHolder must be
independent of any realm. These steps may throw an exception if transferral is not possible. A set of steps that receives the data in dataHolder, using it to set up
value as appropriate. value will be a newly-created instance of the
platform object type in question, with none of its internal data set up; setting
that up is the job of these steps. These steps may throw an exception if it is not possible to receive the transfer. It is up to the definition of individual platform objects to determine what data is transferred
by these steps. Typically the steps are very symmetric. The For a given platform object, only the object's primary interface is
considered during the transferring process. Thus, if inheritance is involved in defining the
interface, each Platform objects that are transferable
objects have a [[Detached]] internal slot. This
is used to ensure that once a platform object has been transferred, it cannot be transferred
again. Objects defined in the JavaScript specification are handled by the
StructuredSerializeWithTransfer abstract operation directly. The StructuredSerializeInternal abstract operation takes as input a JavaScript
value value and serializes it to a realm-independent form, represented
here as a Record. This serialized form has all the information necessary to later
deserialize into a new JavaScript value in a different realm. This process can throw an exception, for example when trying to serialize un-serializable
objects. If memory was not supplied, let memory be an empty map. The purpose of the memory map is to avoid serializing objects twice.
This ends up preserving cycles and the identity of duplicate objects in graphs. If memory[value] exists, then
return memory[value]. Let deep be false. If Type(value) is Undefined, Null, Boolean,
Number, BigInt, or String, then return { [[Type]]: "primitive",
[[Value]]: value }. If Type(value) is Symbol, then throw a
" If value has a [[BooleanData]] internal slot, then set serialized to
{ [[Type]]: "Boolean", [[BooleanData]]: value.[[BooleanData]] }. Otherwise, if value has a [[NumberData]] internal slot, then set
serialized to { [[Type]]: "Number", [[NumberData]]: value.[[NumberData]]
}. Otherwise, if value has a [[BigIntData]]
internal slot, then set serialized to { [[Type]]: "BigInt", [[BigIntData]]:
value.[[BigIntData]] }. Otherwise, if value has a [[StringData]] internal slot, then set
serialized to { [[Type]]: "String", [[StringData]]: value.[[StringData]]
}. Otherwise, if value has a [[DateValue]] internal slot, then set
serialized to { [[Type]]: "Date", [[DateValue]]: value.[[DateValue]]
}. Otherwise, if value has a [[RegExpMatcher]] internal slot, then set
serialized to { [[Type]]: "RegExp", [[RegExpMatcher]]:
value.[[RegExpMatcher]], [[OriginalSource]]: value.[[OriginalSource]],
[[OriginalFlags]]: value.[[OriginalFlags]] }. Otherwise, if value has an [[ArrayBufferData]] internal slot, then: If IsSharedArrayBuffer(value) is true, then:
If the current settings object's cross-origin isolated
capability is false, then throw a " If forStorage is true, then throw a
" Otherwise, set serialized to { [[Type]]: "SharedArrayBuffer",
[[ArrayBufferData]]: value.[[ArrayBufferData]], [[ArrayBufferByteLength]]:
value.[[ArrayBufferByteLength]], [[AgentCluster]]: the surrounding
agent's agent cluster }. Otherwise: If IsDetachedBuffer(value) is true, then throw a
" Let dataCopy be ? CreateByteDataBlock(size). This can throw a Perform CopyDataBlockBytes(dataCopy, 0,
value.[[ArrayBufferData]], 0, size). If value has an [[ArrayBufferMaxByteLength]] internal slot, then set
serialized to { [[Type]]: "ResizableArrayBuffer", [[ArrayBufferData]]:
dataCopy, [[ArrayBufferByteLength]]: size,
[[ArrayBufferMaxByteLength]]: value.[[ArrayBufferMaxByteLength]] }. Otherwise, set serialized to { [[Type]]: "ArrayBuffer",
[[ArrayBufferData]]: dataCopy, [[ArrayBufferByteLength]]: size
}. Otherwise, if value has a [[ViewedArrayBuffer]] internal slot, then: If IsArrayBufferViewOutOfBounds(value) is true, then throw a
" Let bufferSerialized be ?
StructuredSerializeInternal(buffer, forStorage,
memory). Assert: bufferSerialized.[[Type]] is "ArrayBuffer",
"ResizableArrayBuffer", "SharedArrayBuffer", or "GrowableSharedArrayBuffer". If value has a [[DataView]] internal slot, then set serialized to
{ [[Type]]: "ArrayBufferView", [[Constructor]]: "DataView", [[ArrayBufferSerialized]]:
bufferSerialized, [[ByteLength]]: value.[[ByteLength]], [[ByteOffset]]:
value.[[ByteOffset]] }. Otherwise: Assert: value has a [[TypedArrayName]] internal slot. Set serialized to { [[Type]]: "ArrayBufferView", [[Constructor]]:
value.[[TypedArrayName]], [[ArrayBufferSerialized]]: bufferSerialized,
[[ByteLength]]: value.[[ByteLength]], [[ByteOffset]]:
value.[[ByteOffset]], [[ArrayLength]]: value.[[ArrayLength]] }. Otherwise, if value has [[MapData]] internal slot, then: Set serialized to { [[Type]]: "Map", [[MapData]]: a new empty List }. Set deep to true. Otherwise, if value has [[SetData]] internal slot, then: Set serialized to { [[Type]]: "Set", [[SetData]]: a new empty List }. Set deep to true. Otherwise, if value has an [[ErrorData]] internal slot and value is not
a platform object, then: Let name be ? Get(value,
"name"). If name is not one of "Error", "EvalError", "RangeError", "ReferenceError",
"SyntaxError", "TypeError", or "URIError", then set name to "Error". Let valueMessageDesc be ? value.[[GetOwnProperty]](" Let message be undefined if
IsDataDescriptor(valueMessageDesc) is false, and
? ToString(valueMessageDesc.[[Value]]) otherwise. Set serialized to { [[Type]]: "Error", [[Name]]: name,
[[Message]]: message }. User agents should attach a serialized representation of any interesting accompanying
data which are not yet specified, notably the See the Error Stacks proposal for in-progress work on specifying
this data. [JSERRORSTACKS] Otherwise, if value is an Array exotic object, then: Let valueLenDescriptor be ?
OrdinaryGetOwnProperty(value, " Let valueLen be valueLenDescriptor.[[Value]]. Set serialized to { [[Type]]: "Array", [[Length]]: valueLen,
[[Properties]]: a new empty List }. Set deep to true. Otherwise, if value is a platform object that is a serializable object: If value has a [[Detached]] internal slot whose value is true,
then throw a " Set serialized to { [[Type]]: typeString }. Set deep to true. Otherwise, if value is a platform object, then throw a
" Otherwise, if value is an exotic object and value is not the
%Object.prototype% intrinsic object associated with any realm, then
throw a " Otherwise: Set serialized to { [[Type]]: "Object", [[Properties]]: a new empty List }. Set deep to true. %Object.prototype% will end up being handled via this step and
subsequent steps. The end result is that its exoticness is ignored, and after deserialization
the result will be an empty object (not an immutable prototype exotic object).
Set memory[value] to
serialized. If deep is true, then: If value has a [[MapData]] internal slot, then:
Let copiedList be a new empty List.
For each Record { [[Key]], [[Value]] }
entry of value.[[MapData]]: For each Record { [[Key]], [[Value]] }
entry of copiedList: Let serializedKey be ?
StructuredSerializeInternal(entry.[[Key]], forStorage,
memory). Let serializedValue be ?
StructuredSerializeInternal(entry.[[Value]], forStorage,
memory). Append { [[Key]]: serializedKey,
[[Value]]: serializedValue } to serialized.[[MapData]]. Otherwise, if value has a [[SetData]] internal slot, then: Let copiedList be a new empty List.
For each entry of value.[[SetData]]: If entry is not the special value empty, append entry to copiedList. For each entry of copiedList: Let serializedEntry be ?
StructuredSerializeInternal(entry, forStorage,
memory). Append serializedEntry to
serialized.[[SetData]]. Otherwise, if value is a platform object that is a serializable object, then perform the serialization
steps for value's primary interface, given value,
serialized, and forStorage. The serialization steps may need to perform a sub-serialization. This is an operation which takes as input a value
subValue, and returns StructuredSerializeInternal(subValue,
forStorage, memory). (In other words, a sub-serialization
is a specialization of StructuredSerializeInternal to be consistent within this
invocation.) Otherwise, for each key in !
EnumerableOwnProperties(value, key): If ! HasOwnProperty(value, key) is true, then: Let inputValue be ? value.[[Get]](key,
value). Let outputValue be ?
StructuredSerializeInternal(inputValue, forStorage,
memory). Append { [[Key]]: key, [[Value]]:
outputValue } to serialized.[[Properties]]. Return serialized. It's important to realize that the Records
produced by StructuredSerializeInternal might contain "pointers" to other records
that create circular references. For example, when we pass the following JavaScript object into
StructuredSerializeInternal: it produces the following result: Return ? StructuredSerializeInternal(value, false). Return ? StructuredSerializeInternal(value, true). The StructuredDeserialize abstract operation takes as input a Record
serialized, which was previously produced by StructuredSerialize or
StructuredSerializeForStorage, and deserializes it into a new JavaScript value,
created in targetRealm. This process can throw an exception, for example when trying to allocate memory for the new
objects (especially If memory was not supplied, let memory be an empty map. The purpose of the memory map is to avoid deserializing objects
twice. This ends up preserving cycles and the identity of duplicate objects in graphs. If memory[serialized] exists, then
return memory[serialized]. Let deep be false. Let value be an uninitialized value. If serialized.[[Type]] is "primitive", then set value to
serialized.[[Value]]. Otherwise, if serialized.[[Type]] is "Boolean", then set value to a
new Boolean object in targetRealm whose [[BooleanData]] internal slot value is
serialized.[[BooleanData]]. Otherwise, if serialized.[[Type]] is "Number", then set value to a
new Number object in targetRealm whose [[NumberData]] internal slot value is
serialized.[[NumberData]]. Otherwise, if serialized.[[Type]] is "BigInt", then set value to a
new BigInt object in targetRealm whose [[BigIntData]] internal slot value is
serialized.[[BigIntData]]. Otherwise, if serialized.[[Type]] is "String", then set value to a
new String object in targetRealm whose [[StringData]] internal slot value is
serialized.[[StringData]]. Otherwise, if serialized.[[Type]] is "Date", then set value to a new
Date object in targetRealm whose [[DateValue]] internal slot value is
serialized.[[DateValue]]. Otherwise, if serialized.[[Type]] is "RegExp", then set value to a
new RegExp object in targetRealm whose [[RegExpMatcher]] internal slot value is
serialized.[[RegExpMatcher]], whose [[OriginalSource]] internal slot value is
serialized.[[OriginalSource]], and whose [[OriginalFlags]] internal slot value is
serialized.[[OriginalFlags]]. Otherwise, if serialized.[[Type]] is "SharedArrayBuffer", then: If targetRealm's corresponding agent cluster is not
serialized.[[AgentCluster]], then then throw a
" Otherwise, if serialized.[[Type]] is "GrowableSharedArrayBuffer", then: If targetRealm's corresponding agent cluster is not
serialized.[[AgentCluster]], then then throw a
" Otherwise, if serialized.[[Type]] is "ArrayBuffer", then set value to a
new ArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value
is serialized.[[ArrayBufferData]], and whose [[ArrayBufferByteLength]] internal slot
value is serialized.[[ArrayBufferByteLength]]. If this throws an exception, catch it, and then throw a
" Otherwise, if serialized.[[Type]] is "ResizableArrayBuffer", then set
value to a new ArrayBuffer object in targetRealm whose [[ArrayBufferData]]
internal slot value is serialized.[[ArrayBufferData]], whose
[[ArrayBufferByteLength]] internal slot value is
serialized.[[ArrayBufferByteLength]], and whose [[ArrayBufferMaxByteLength]] internal
slot value is a serialized.[[ArrayBufferMaxByteLength]]. If this throws an exception, catch it, and then throw a
" Otherwise, if serialized.[[Type]] is "ArrayBufferView", then: Let deserializedArrayBuffer be ?
StructuredDeserialize(serialized.[[ArrayBufferSerialized]],
targetRealm, memory). If serialized.[[Constructor]] is "DataView", then set value to a
new DataView object in targetRealm whose [[ViewedArrayBuffer]] internal slot value
is deserializedArrayBuffer, whose [[ByteLength]] internal slot value is
serialized.[[ByteLength]], and whose [[ByteOffset]] internal slot value is
serialized.[[ByteOffset]]. Otherwise, set value to a new typed array object in targetRealm,
using the constructor given by serialized.[[Constructor]], whose
[[ViewedArrayBuffer]] internal slot value is deserializedArrayBuffer, whose
[[TypedArrayName]] internal slot value is serialized.[[Constructor]], whose
[[ByteLength]] internal slot value is serialized.[[ByteLength]], whose
[[ByteOffset]] internal slot value is serialized.[[ByteOffset]], and whose
[[ArrayLength]] internal slot value is serialized.[[ArrayLength]]. Otherwise, if serialized.[[Type]] is "Map", then: Set value to a new Map object in targetRealm whose [[MapData]]
internal slot value is a new empty List. Set deep to true. Otherwise, if serialized.[[Type]] is "Set", then: Set value to a new Set object in targetRealm whose [[SetData]]
internal slot value is a new empty List. Set deep to true. Otherwise, if serialized.[[Type]] is "Array", then: Let outputProto be
targetRealm.[[Intrinsics]].[[%Array.prototype%]]. Set value to ! ArrayCreate(serialized.[[Length]],
outputProto). Set deep to true. Otherwise, if serialized.[[Type]] is "Object", then: Set value to a new Object in targetRealm. Set deep to true. Otherwise, if serialized.[[Type]] is "Error", then: Let prototype be %Error.prototype%. If serialized.[[Name]] is "EvalError", then set prototype to
%EvalError.prototype%. If serialized.[[Name]] is "RangeError", then set prototype
to %RangeError.prototype%. If serialized.[[Name]] is "ReferenceError", then set
prototype to %ReferenceError.prototype%. If serialized.[[Name]] is "SyntaxError", then set prototype
to %SyntaxError.prototype%. If serialized.[[Name]] is "TypeError", then set prototype to
%TypeError.prototype%. If serialized.[[Name]] is "URIError", then set prototype to
%URIError.prototype%. Let message be serialized.[[Message]]. Set value to OrdinaryObjectCreate(prototype, «
[[ErrorData]] »). Let messageDesc be PropertyDescriptor{ [[Value]]:
message, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true
}. If message is not undefined, then perform !
OrdinaryDefineOwnProperty(value, " Any interesting accompanying data attached to serialized should be
deserialized and attached to value. Otherwise: Let interfaceName be serialized.[[Type]]. If the interface identified by interfaceName is not
exposed in targetRealm, then throw a
" Set deep to true. Set memory[serialized] to
value. If deep is true, then: If serialized.[[Type]] is "Map", then: For each Record { [[Key]], [[Value]] }
entry of serialized.[[MapData]]: Let deserializedKey be ?
StructuredDeserialize(entry.[[Key]], targetRealm,
memory). Let deserializedValue be ?
StructuredDeserialize(entry.[[Value]], targetRealm,
memory). Append { [[Key]]: deserializedKey,
[[Value]]: deserializedValue } to value.[[MapData]]. Otherwise, if serialized.[[Type]] is "Set", then: For each entry of
serialized.[[SetData]]: Let deserializedEntry be ?
StructuredDeserialize(entry, targetRealm,
memory). Append deserializedEntry to
value.[[SetData]]. Otherwise, if serialized.[[Type]] is "Array" or "Object", then: For each Record { [[Key]], [[Value]] }
entry of serialized.[[Properties]]: Let deserializedValue be ?
StructuredDeserialize(entry.[[Value]], targetRealm,
memory). Let result be ! CreateDataProperty(value,
entry.[[Key]], deserializedValue). Assert: result is true. Otherwise: Perform the appropriate deserialization steps for the interface identified
by serialized.[[Type]], given serialized, value, and
targetRealm. The deserialization steps may need to perform a sub-deserialization. This is an operation which takes as input a
previously-serialized Record subSerialized, and returns
StructuredDeserialize(subSerialized, targetRealm,
memory). (In other words, a sub-deserialization is a specialization
of StructuredDeserialize to be consistent within this invocation.) Return value. Let memory be an empty map. In addition to how it is used normally by
StructuredSerializeInternal, in this algorithm memory is also used to
ensure that StructuredSerializeInternal ignores items in transferList,
and let us do our own handling instead. For each transferable of
transferList: If transferable has neither an [[ArrayBufferData]] internal slot nor a
[[Detached]] internal slot, then throw a
" transferable is not transferred yet as transferring has side
effects and StructuredSerializeInternal needs to be able to throw first. Let serialized be ? StructuredSerializeInternal(value,
false, memory). Let transferDataHolders be a new empty List. For each transferable of
transferList: If transferable has an [[ArrayBufferData]] internal slot and
IsDetachedBuffer(transferable) is true, then throw a
" If transferable has an [[ArrayBufferData]] internal slot, then: If transferable has an [[ArrayBufferMaxByteLength]] internal slot, then: Set dataHolder.[[Type]] to "ResizableArrayBuffer". Set dataHolder.[[ArrayBufferData]] to
transferable.[[ArrayBufferData]]. Set dataHolder.[[ArrayBufferByteLength]] to
transferable.[[ArrayBufferByteLength]]. Set dataHolder.[[ArrayBufferMaxByteLength]] to
transferable.[[ArrayBufferMaxByteLength]]. Otherwise: Set dataHolder.[[Type]] to "ArrayBuffer". Set dataHolder.[[ArrayBufferData]] to
transferable.[[ArrayBufferData]]. Set dataHolder.[[ArrayBufferByteLength]] to
transferable.[[ArrayBufferByteLength]]. Perform ? DetachArrayBuffer(transferable). Specifications can use the [[ArrayBufferDetachKey]] internal slot to prevent
Otherwise: Assert: transferable is a platform object that is
a transferable object. Let interfaceName be the identifier of the primary interface
of transferable. Set dataHolder.[[Type]] to interfaceName. Perform the appropriate transfer steps for the interface identified by
interfaceName, given transferable and dataHolder. Set transferable.[[Detached]] to true. Append dataHolder to
transferDataHolders. Return { [[Serialized]]: serialized, [[TransferDataHolders]]:
transferDataHolders }. Let memory be an empty map. Analogous to StructuredSerializeWithTransfer, in addition to how it
is used normally by StructuredDeserialize, in this algorithm memory is
also used to ensure that StructuredDeserialize ignores items in
serializeWithTransferResult.[[TransferDataHolders]], and let us do our own handling
instead. Let transferredValues be a new empty List. For each transferDataHolder of
serializeWithTransferResult.[[TransferDataHolders]]: Let value be an uninitialized value. If transferDataHolder.[[Type]] is "ArrayBuffer", then set value to a
new ArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value
is transferDataHolder.[[ArrayBufferData]], and whose [[ArrayBufferByteLength]]
internal slot value is transferDataHolder.[[ArrayBufferByteLength]]. In cases where the original memory occupied by [[ArrayBufferData]] is
accessible during the deserialization, this step is unlikely to throw an exception, as no new
memory needs to be allocated: the memory occupied by [[ArrayBufferData]] is instead just
getting transferred into the new ArrayBuffer. This could be true, for example, when both the
source and target realms are in the same process. Otherwise, if transferDataHolder.[[Type]] is "ResizableArrayBuffer", then set
value to a new ArrayBuffer object in targetRealm whose
[[ArrayBufferData]] internal slot value is transferDataHolder.[[ArrayBufferData]],
whose [[ArrayBufferByteLength]] internal slot value is
transferDataHolder.[[ArrayBufferByteLength]], and whose
[[ArrayBufferMaxByteLength]] internal slot value is
transferDataHolder.[[ArrayBufferMaxByteLength]]. For the same reason as the previous step, this step is also unlikely to throw
an exception. Otherwise: Let interfaceName be transferDataHolder.[[Type]]. If the interface identified by interfaceName is not exposed in
targetRealm, then throw a " Perform the appropriate transfer-receiving steps for the interface
identified by interfaceName given transferDataHolder and
value. Set memory[transferDataHolder] to
value. Append value to
transferredValues. Let deserialized be ?
StructuredDeserialize(serializeWithTransferResult.[[Serialized]],
targetRealm, memory). Return { [[Deserialized]]: deserialized, [[TransferredValues]]:
transferredValues }. Other specifications may use the abstract operations defined here. The following provides
some guidance on when each abstract operation is typically useful, with examples. Cloning a value to another realm, with a transfer list, but where the target
realm is not known ahead of time. In this case the serialization step can be performed
immediately, with the deserialization step delayed until the target realm becomes known. Creating a realm-independent snapshot of a given value which can be saved for an
indefinite amount of time, and then reified back into a JavaScript value later, possibly
multiple times. StructuredSerializeForStorage can be used for situations where the serialization
is anticipated to be stored in a persistent manner, instead of passed between realms. It throws
when attempting to serialize Any API for persisting JavaScript values to the filesystem would also use
StructuredSerializeForStorage on its input and StructuredDeserialize
on its output. In general, call sites may pass in Web IDL values instead of JavaScript values; this is to be
understood to perform an implicit conversion to the
JavaScript value before invoking these algorithms. Call sites that are not invoked as a result of author code synchronously calling into a user
agent method must take care to properly prepare to run script and prepare to
run a callback before invoking StructuredSerialize,
StructuredSerializeForStorage, or StructuredSerializeWithTransfer
abstract operations, if they are being performed on arbitrary objects. This is necessary because
the serialization process can invoke author-defined accessors as part of its final
deep-serialization steps, and these accessors could call into operations that rely on the entry and incumbent concepts being properly set up. In contrast, a hypothetical API that used StructuredSerialize to
serialize some author-supplied object periodically, directly from a task on the event loop, would need to ensure it performs
the appropriate preparations beforehand. As of this time, we know of no such APIs on the platform;
usually it is simpler to perform the serialization ahead of time, as a synchronous consequence of
author code. Takes the input value and returns a deep copy by performing the structured clone algorithm.
Transferable objects listed in the Throws a " Support in all current engines. The Let serialized be ?
StructuredSerializeWithTransfer(value, options[" Let deserializeRecord be ?
StructuredDeserializeWithTransfer(serialized, this's
relevant realm). Return deserializeRecord.[[Deserialized]]. Every XML and HTML document in an HTML UA is represented by a The Interactive user agents typically expose the The When a The document's referrer is a string (representing a URL) that
can be set when the Support in all current engines. DOM defines a Each Each Each Each Each Each As the name indicates, this is used for interfacing with the WebDriver
BiDi specification, which needs to be informed about certain occurrences during the early
parts of the Each This is only populated for " Each This is intended to suppress firing of DOM Mutation Events in cases when they
would normally fire. The specification describing mutation events is not actively maintained so
it does not look at this flag, but implementations are expected to act as though it did.
[UIEVENTS] DOM defines the Support in all current engines. Returns the URL of the The The Returns the HTTP cookies that apply to the Can be set, to add a new cookie to the element's set of HTTP cookies. If the contents are sandboxed into an
opaque origin (e.g., in an Support in all current engines. The A
On setting, if the document is a cookie-averse Since the The
html element, which is the
element always found in that position in HTML documents. It contains two elements,
head and body, as well as a
head start tag ends up being dropped silently,
and all the whitespace after the body end tag ends up placed at the end of the
body.head element contains a title element, which itself contains a
body element
contains an h1 element, a p element, and a comment.
script element or using event
handler content attributes. For example, here is a form with a script that sets the value
of the form's output element to say "Hello World":a element in the tree above)
can have its "href" attribute changed in several
ways:
1.10.1 Writing secure applications with HTML
https://example.com/message.cgi?say=%3Cscript%3Ealert%28%27Oh%20no%21%27%29%3C/script%3E
img, it is important to safelist
any provided attributes as well. If one allowed all attributes then an attacker could, for
instance, use the onload attribute to run arbitrary
script.javascript:", but user agents can
implement (and indeed, have historically implemented) others.base element to be inserted means any script elements
in the page with relative links can be hijacked, and similarly that any form submissions can
get redirected to a hostile site.
iframe and then convinces the user to click, for instance by having the user
play a reaction game. Once the user is playing the game, the hostile site can quickly position
the iframe under the mouse cursor just as the user is about to click, thus tricking the user
into clicking the victim site's interface.window object to the value of the top
attribute).1.10.2 Common pitfalls to avoid when using the scripting APIs
img elements and the load event. The event could fire as soon as the element has been
parsed, especially if the image has already been cached (which is common).onload handler on an
img element to catch the load event:img element and then in a separate
script added the event listeners, there's a chance that the load
event would be fired in between, leading it to be missed:1.10.3 How to catch mistakes when writing HTML: validators and conformance checkers
1.11.1 Presentational markup
<font color=""> throughout requires changes across the entire site,
whereas a similar change to a site based on CSS can be done by changing a single file.style attribute and the style element. Use of the style attribute is somewhat discouraged in production environments, but
it can be useful for rapid prototyping (where its rules can be directly moved into a separate
style sheet later) and for providing specific styles in unusual cases where a separate style sheet
would be inconvenient. Similarly, the style element can be useful in syndication or
for page-specific styles, but in general an external style sheet is likely to be more convenient
when the styles apply to multiple pages.b, i,
hr, s, small, and u.1.11.2 Syntax errors
<table><hr>... example mentioned above, are incompatible with streaming
user agents (user agents that process HTML files in one pass, without storing state). To avoid
interoperability problems with such user agents, any syntax resulting in such behavior is
considered invalid.
i elements have to be reconstructed in each paragraph, resulting in progressively
more elements in each paragraph:
?bill&ted":?art©", not the intended "?art©",
because even without the final semicolon, "©" is handled the same
as "©" and thus gets interpreted as "©":
<capton> instead of <caption>, this would be flagged as an error and the author could correct
the typo immediately.
1.11.3 Restrictions on content models and on attribute values
section
element inside a kbd element, since it is highly unlikely for an author to indicate
that an entire section should be keyed in.
ul element, which only allows li element children. Lists by definition
consist just of zero or more list items, so if a ul element contains something
other than an li element, it's not clear what was meant.div elements are rendered as block boxes, and span elements as inline boxes. Putting a block box in an
inline box is unnecessarily confusing; since either nesting just div
elements, or nesting just span elements, or nesting span elements
inside div elements all serve the same purpose as nesting a div
element in a span element, but only the latter involves a block box in
an inline box, the latter combination is disallowed.button element cannot contain a textarea
element. This is because the default behavior of such nesting interactive elements would be
highly confusing to users. Instead of nesting these elements, they can be placed side by
side.disabled
attribute to the value "false" is disallowed, because despite the
appearance of meaning that the element is enabled, it in fact means that the element is
disabled (what matters for implementations is the presence of the attribute, not its
value).area element's shape attribute, despite accepting both circ and circle values in practice as synonyms, disallows
the use of the circ value, so as to simplify
tutorials and other learning aids. There would be no benefit to allowing both, but it would
cause extra confusion when teaching the language.
form element isn't allowed inside phrasing content,
because when parsed as HTML, a form element's start tag will imply a
p element's end tag. Thus, the following markup results in two paragraphs, not one:id attributes with the same value. Duplicate IDs lead to the wrong
element being selected, with sometimes disastrous effects whose cause is hard to determine.script element's src attribute causes the element's contents to be ignored.
However, this isn't obvious, especially if the element's contents appear to be executable script
— which can lead to authors spending a lot of time trying to debug the inline script
without realizing that it is not executing. To reduce this problem, this specification makes it
non-conforming to have executable script in a script element when the src attribute is present. This means that authors who are
validating their documents are less likely to waste time with this kind of mistake.lang and xml:lang attributes
intended to keep the two synchronized.xmlns attributes in the HTML serialization, which are intended to ensure that
elements in conforming documents end up in the same namespaces whether processed as HTML or
XML.target attribute that start with an U+005F LOW LINE
character (_) to only specific predefined values allows new predefined values to be introduced
at a future time without conflicting with author-defined values.1.12 Suggested reading
2 Common infrastructure
2.1 Terminology
Document
objects and their descendant DOM trees, and to serialized byte streams using the HTML syntax or the XML syntax, depending
on context.Document objects
can find themselves in. [DOM] (Such uses are always hyperlinked to their
definition.)text/html, and the term XML document refers to resources labeled with an XML
MIME type.
2.1.1 Parallelism
2.1.2 Resources
@import
rules, including those indirectly imported by other imported style sheets.2.1.3 XML compatibility
HTML elements" refers to any element in that namespace, even in
XML documents.http://www.w3.org/1999/xhtml"), and all
attributes defined or mentioned in this specification have no namespace.button elements are elements with the element
type button, meaning they have the local name "button" and
(implicitly as defined above) the HTML namespace.Name production defined in XML and they contain no U+003A COLON
characters (:). [XML]2.1.4 DOM trees
length of the text is zero (i.e., not even containing controls or U+0020 SPACE).blur or change events are
fired.popover attribute is not in
the no popover state, then run the hide
popover algorithm given removedNode, false, false, and false.2.1.5 Scripting
Foo object", where Foo is
actually an interface, is sometimes used instead of the more accurate "an object implementing the
interface Foo".2.1.6 Plugins
Document object, but that neither act as child
navigables of the Document nor introduce any Document's DOM.text/plain and
application/octet-stream as having a registered plugin.
Since different users having different sets of plugins provides a
tracking vector that increases the chances of users being uniquely identified, user agents are
encouraged to support the exact same set of plugins for each
user.2.1.7 Character encodings
2.1.8 Conformance classes
foobar element", it would imply that documents are not allowed to
contain elements named foobar.script element in
an XML document, execute the script contained in that element. However, if the element is found
within a transformation expressed in XSLT (assuming the user agent also supports XSLT), then the
processor would instead treat the script element as an opaque element that forms
part of the transform.script element in the example above would still implement the
HTMLScriptElement interface.blockquote element is not a quote,
conformance checkers running without the input of human judgement do not have to check that
blockquote elements only contain quoted material).
address element for
arbitrary contact information; that element can only be used for marking up contact information
for its nearest article or body element ancestor. However, since an
authoring tool is likely unable to determine the difference, an authoring tool is exempt from
that requirement. This does not mean, though, that authoring tools can use address
elements for any block of italics text (for instance); it just means that the authoring tool
doesn't have to verify that when the user uses a tool for inserting contact information for an
article element, that the user really is doing that and not inserting something
else instead.div, b, i, and span and making liberal use
of the style attribute.2.1.9 Dependencies
space in the XML namespace is defined by
Extensible Markup Language (XML). [XML]Name production is defined in XML.
[XML]<?xml-stylesheet?>
processing instruction, defined in Associating Style Sheets with XML documents.
[XMLSSPI]XSLTProcessor
interface and its transformToFragment() and transformToDocument() methods.
[XSLTP]about: scheme
[ABOUT]blob: scheme
[FILEAPI]data: scheme
[RFC2397]http: scheme
[HTTP]https: scheme
[HTTP]mailto:
scheme [MAILTO]sms: scheme
[SMS]urn: scheme
[URN]Accept` headerAccept-Language` headerCache-Control` headerContent-Disposition` headerContent-Language` headerContent-Range` headerLast-Modified` headerRange` headerReferer` headerLink` headerLink` field valueReferrer-Policy` HTTP headerReferrer-Policy` header algorithmno-referrer",
"no-referrer-when-downgrade",
"origin-when-cross-origin", and
"unsafe-url" referrer policiesNavigationTimingType and its
"navigate",
"reload", and
"back_forward" values.PerformanceEntry and its
name,
entryType,
startTime, and
duration attributes.Date object
representing a particular time (which could be the special value Not-a-Number), the milliseconds
component of that time, if any, must be truncated to an integer, and the time value of the newly
created Date object must represent the resulting truncated time.Date object
created representing that time would represent the same time as that created representing the
time 2000-01-01T00:00:00.023Z, 45 millionths earlier. If the given time is NaN, then the result
is a Date object that represents a time value NaN (indicating that the object does
not represent a specific instant of time).Atomics objectAtomics.waitAsync objectDate classFinalizationRegistry classRegExp classSharedArrayBuffer classSyntaxError classTypeError classRangeError classWeakRef classeval() functionWeakRef.prototype.deref() functionimport()import.metatypeof operatordelete operatorDocumentMouseEvent interfaceMouseEvent interface's relatedTarget attributeMouseEventInit dictionary typeFocusEvent interfaceFocusEvent interface's relatedTarget attributeUIEvent interfaceUIEvent interface's view attributeauxclick eventbeforeinput eventclick eventcontextmenu eventdblclick eventinput eventmousedown eventmouseenter eventmouseleave eventmousemove eventmouseout eventmouseover eventmouseup eventwheel eventkeydown eventkeypress eventkeyup eventTouch interfacetouchend eventPointerEvent interfacePointerEvent interface's pointerType attributepointerdown eventpointerup eventpointercancel eventclick" or "if the event name is keypress". The terms
"name" and "type" for events are synonymous.dialog elements, and also to
define how the Fullscreen API interacts with HTML: [FULLSCREEN]DOMHighResTimeStamp
typedef. [HRT]Blob interface and its
type attributeFile interface and its
name and
lastModified attributesFileList interfaceBlob's snapshot stateMediaStream interfacegetBattery() methodrgb(0,0,0" (with a missing close-parenthesis) for a color value, the close
parenthesis is implied by this error handling rule, and a value is obtained (the color 'black').
However, the similar construct "rgb(0,0," (with both a missing
parenthesis and a missing "blue" value) cannot be parsed, as closing the open construct does not
result in a viable value.
Top
Bottom
Left
Right
Width
'border-top-width'
'border-bottom-width'
'border-left-width'
'border-right-width'
Style
'border-top-style'
'border-bottom-style'
'border-left-style'
'border-right-style'
Color
'border-top-color'
'border-bottom-color'
'border-left-color'
'border-right-color'
Screen interfaceLinkStyle interfaceCSSStyleDeclaration interfacestyle IDL attributecssText attribute of CSSStyleDeclarationStyleSheet interfaceCSSStyleSheet interfaceCSSStyleSheetCSSStyleSheetresize eventscroll eventscrollend eventCanvasRenderingContext2D object's use of fonts depends on the features
described in the CSS Fonts and Font Loading specifications, including
in particular FontFace objects and the font source concept.
[CSSFONTS] [CSSFONTLOAD]DOMMatrix interface, and associated
m11 element,
m12 element,
m21 element,
m22 element,
m41 element, and
m42 elementDOMMatrix2DInit and
DOMMatrixInit dictionariesDOMMatrix from a dictionary
and create a DOMMatrix from a 2D dictionary
algorithms for DOMMatrix2DInit or DOMMatrixInitDOMPointInit dictionary, and associated
x and
y membersWebGLRenderingContext interfaceWebGL2RenderingContext interfaceWebGLContextAttributes dictionaryGPUCanvasContext interfacerole attribute is defined in
Accessible Rich Internet Applications (ARIA), as are the following
roles: [ARIA]aria-* content
attributes are defined in ARIA: [ARIA]ARIAMixin interface, with its associated
ARIAMixin getter steps and
ARIAMixin setter steps hooksreport-uri directiveframe-ancestors directivesandbox directiveSecurityPolicyViolationEvent interfacesecuritypolicyviolation eventPaymentRequest interfaceannotation-xml elementmath elementmerror elementmi elementmn elementmo elementms elementmtext elementSVGElement interfaceSVGImageElement interfaceSVGScriptElement interfaceSVGSVGElement interfacea elementdesc elementforeignObject elementimage elementscript elementsvg elementtitle elementuse elementtext-rendering property
2.1.10 Extensibility
typeTime" to a control that returned the time it took the user
to select the current value of a control (say). On the other hand, defining a new control that
appears in a form's elements array would be in violation
of the above requirement, as it would violate the definition of elements given in this specification.
2.1.11 Interactions with XPath and XSLT
document.evaluate() API) must act as if the following edit was applied
to the XPath 1.0 specification.
xmlns is not used: if the QName does not have a prefix, then the
namespace URI is null (this is the same way attribute names are expanded). It is an error if the
QName has a prefix for which there is
no namespace declaration in the expression context.
interactive" and then to "complete" if they are aborted.
script element section, and of
XSLT, XPath, and HTML in the template element
section.2.2 Policy-controlled features
Firefox🔰 74+SafariNoChrome🔰 88+
Opera?Edge🔰 88+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome AndroidNoWebView Android?Samsung Internet?Opera Android?Firefox🔰 74+SafariNoChrome64+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?Firefox🔰 74+SafariNoChrome88+
Opera?Edge88+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?autoplay", which has a default allowlist of 'self'.cross-origin-isolated", which has a default allowlist of 'self'.2.3 Common microsyntaxes
2.3.1 Common parser idioms
2.3.2 Boolean attributes
checked and disabled
attributes are the boolean attributes.2.3.3 Keywords and enumerated attributes
2.3.4 Numbers
2.3.4.1 Signed integers
+" is
ignored, but it is not conforming.)2.3.4.2 Non-negative integers
2.3.4.3 Floating-point numbers
max attribute of the progress element). However, in
some cases the user agent requirements include checking if a string is a valid
floating-point number (e.g., the value sanitization algorithm for the Number state of the input element, or the
parse a srcset attribute algorithm).+"
is ignored, but it is not conforming.)2.3.4.4 Percentages and lengths
2.3.4.5 Nonzero percentages and lengths
2.3.4.6 Lists of floating-point numbers
2.3.4.7 Lists of dimensions
2.3.5 Dates and times
time element.2.3.5.1 Months
2.3.5.2 Dates
2.3.5.3 Yearless dates
02",
meaning February, then the day can be 29, as if the year was a leap year.2.3.5.4 Times
2.3.5.5 Local dates and times
2.3.5.6 Time zones
2.3.5.7 Global dates and times
0037-12-13 00:00Z"1979-10-14T12:00:00.001-04:00"8592-01-01T02:09+02:09"T" is replaced by a space, it must be a single space
character. The string "2001-12-21 12:00Z" (with two spaces
between the components) would not be parsed successfully.2.3.5.8 Weeks
2.3.5.9 Durations
2.3.5.10 Vaguer moments in time
2.3.6 Colors
transparent", then return an error.00".
2.3.7 Space-separated tokens
2.3.8 Comma-separated tokens
a ,b,,d d " consists of four tokens: "a", "b", the empty
string, and "d d". Leading and trailing whitespace around each token doesn't count as part of
the token, and the empty string can be a token.2.3.9 References
name attribute of an element with type type in
the same tree.id or name attribute whose value is s, or null if there is no such
element.id attributes are accounted for when
parsing, they are not used in determining whether a value is a valid hash-name
reference. That is, a hash-name reference that refers to an element based on id is a conformance error (unless that element also has a name attribute with the same value).2.3.10 Media queries
<media-query-list> production of Media Queries.
[MQ]2.3.11 Unique internal values
2.4 URLs
2.4.1 Terminology
about:legacy-compat as a reserved,
though unresolvable, about: URL, for use in DOCTYPEs in HTML documents when needed for
compatibility with XML tools. [ABOUT]about:html-kind as a reserved,
though unresolvable, about: URL, that is used as an
identifier for kinds of media tracks. [ABOUT]about:srcdoc as a reserved, though
unresolvable, about: URL, that is used as the URL of iframe srcdoc documents.
[ABOUT]Document object document is the
URL record obtained by running these steps:iframe
srcdoc document, then:about:blank and document's about base URL is non-null, then return
document's about base URL.Document object is the
URL record obtained by running these steps:base element that has an href attribute in the Document, then return the
Document's fallback base URL.base element
in the Document that has an href attribute, in
tree order.
about:blank if its scheme is "about", its path contains a single string "blank", its
username and password are the empty string, and its host is null.about:blank?foo#bar" matches about:blank.about:srcdoc if its scheme is "about", its path contains a single string "srcdoc",
its query is null, its username and password are the empty string, and its host is null.about:srcdoc ensures that the
URL's query is null is because it is not
possible to create an iframe srcdoc document whose URL has a non-null query, unlike Documents whose URL matches about:blank. In other
words, the set of all URLs that match
about:srcdoc only vary in their fragment.2.4.2 Parsing URLs
Document object or environment settings object environment,
run these steps. They return failure or a URL.Document object; otherwise
environment's API base URL.Document object or environment
settings object environment, run these steps. They return failure or a
URL.Document object, then set encoding
to environment's character
encoding.Window object, set encoding to environment's relevant
global object's associated
Document's character
encoding.Document object; otherwise
environment's API base URL.Document object or
environment settings object environment, run these steps. They return
failure or a string.2.4.3 Dynamic changes to base URLs
href attribute's value should be reparsed, relative to the element's node
document and the UI updated appropriately.:link/:visited pseudo-classes
might have been affected.ping attribute and its
URL(s) are being shown to the user, then the ping attribute's tokens should be reparsed, relative to the element's node
document and the UI updated appropriately.q, blockquote, ins, or
del element with a cite attributecite attribute is being
shown to the user, or if any data derived from that URL is affecting the display,
then the cite attribute's value should be reparsed, relative to the element's node document and the UI updated
appropriately.img elements, although subsequent accesses of the src IDL attribute from script will return a new absolute
URL that might no longer correspond to the image being shown.2.5 Fetching resources
2.5.1 Terminology
basic", "cors", or "default" is CORS-same-origin.
[FETCH]opaque" or "opaqueredirect" is CORS-cross-origin.no-cors" if corsAttributeState
is No CORS, and "cors"
otherwise.no-cors", set mode to "same-origin".include".same-origin".2.5.2 Determining the type of a resource
2.5.3 Extracting character encodings from
meta elementsmeta element,
given a string s, is as follows. It either returns a character encoding or
nothing.charset". If no such match is found, return nothing.charset" (there might not be any).2.5.4 CORS settings attributes
Firefox8+Safari6+Chrome13+
Opera?Edge79+
Edge (Legacy)12+Internet ExplorerYes
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android? State
Keywords
Brief description
Anonymous
anonymous
Requests for the element will have their
mode set to " cors" and their
credentials mode set to "same-origin".
(the empty string)
Use Credentials
use-credentials
Requests for the element will have their mode set to " cors" and their credentials mode set to "include".
anonymous keyword.cors", certain CORS settings attributes have been repurposed to have a
slightly different meaning, wherein they only impact the request's credentials mode. To perform this translation, we
define the CORS settings attribute credentials mode for a given CORS
settings attribute to be determined by switching on the attribute's state:same-origin"include"2.5.5 Referrer policy attributes
noreferrer link
type;meta element with name attribute set to referrer.Referrer-Policy` HTTP
header.2.5.6 Nonce attributes
Firefox31+SafariYesChromeYes
Opera?EdgeYes
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?nonce content
attribute represents a cryptographic nonce ("number used once") which can be used by Content
Security Policy to determine whether or not a given fetch will be allowed to proceed. The
value is text. [CSP]nonce content attribute ensure that the
cryptographic nonce is only exposed to script (and not to side-channels like CSS attribute
selectors) by taking the value from the content attribute, moving it into an internal slot
named [[CryptographicNonce]], exposing it to script
via the HTMLOrSVGElement interface mixin, and setting the content attribute to the
empty string. Unless otherwise specified, the slot's value is the empty string.element.noncenonce
content attribute.element.nonce = valueFirefox75+Safari🔰 10+Chrome61+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?nonce IDL attribute must, on getting, return the
value of this element's [[CryptographicNonce]]; and on setting, set this element's
[[CryptographicNonce]] to the given value.nonce IDL attribute does not update the corresponding
content attribute. This, as well as the below setting of the nonce content attribute to the empty string when an element
becomes browsing-context connected, is meant to prevent exfiltration of the nonce
value through mechanisms that can easily read content attributes, such as selectors. Learn more in
issue #2369, where this behavior was
introduced.nonce content attribute:
HTMLOrSVGElement, then
return.nonce or
namespace is not null, then return.HTMLOrSVGElement
becomes browsing-context connected, the user agent must execute the following steps
on the element:nonce content attribute
attr whose value is not the empty string, then:nonce" and the empty
string.HTMLOrSVGElement must set the
[[CryptographicNonce]] slot on the copy to the value of the slot on the element being
cloned.2.5.7 Lazy loading attributes
Firefox75+Safari15.4+Chrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?Keyword
State
Description
lazy
Lazy
Used to defer fetching a resource until some conditions are met.
eager
Eager
Used to fetch a resource immediately; the default state.
img and iframe element has associated lazy load resumption
steps, initially null.img and iframe elements that will lazy load, these steps are run from the lazy load
intersection observer's callback or when their lazy loading attribute is set
to the Eager state. This causes the element to
continue loading.Document has a lazy load intersection observer, initially set to
null but can be set to an IntersectionObserver instance.IntersectionObserver instance, initialized as follows:IntersectionObserver constructor. However, we're forced to use the
JavaScript-exposed constructor in this specification, until Intersection Observer
exposes low-level hooks for use in specifications. See bug w3c/IntersectionObserver#464
which tracks this. [INTERSECTIONOBSERVER]isIntersecting is true, then
set resumptionSteps to entry.target's
lazy load resumption steps.target.target's lazy load resumption
steps to null.isIntersecting and
target getters. See w3c/IntersectionObserver#464.
[INTERSECTIONOBSERVER]IntersectionObserverInit dictionary with the
following dictionary members: «[ "rootMargin" → lazy load root
margin ]»IntersectionObserver API does not support changing the root
margin. See issue w3c/IntersectionObserver#428.observe method with element as the
argument.observe method. See w3c/IntersectionObserver#464.
[INTERSECTIONOBSERVER]unobserve method with element as
the argument.unobserve method. See w3c/IntersectionObserver#464.
[INTERSECTIONOBSERVER]
The lazy load root margin is an implementation-defined value, but with
the following suggestions to consider:2.5.8 Blocking attributes
Possible blocking token
Description
" render"
The element is potentially render-blocking.
render", or if it is
implicitly potentially render-blocking, which will be defined at the individual
elements. By default, an element is not implicitly potentially render-blocking.2.5.9 Fetch priority attributes
Keyword
State
Description
high
high
Signals a high-priority fetch relative to other
resources with the same destination.
low
low
Signals a low-priority fetch relative to other
resources with the same destination.
auto
auto
Signals automatic determination of fetch priority
relative to other resources with the same destination.
2.6 Common DOM interfaces
2.6.1 Reflecting content attributes in IDL attributes
ElementInternals
object. It is typically clear from context and typically identical to the interface of the
reflected IDL attribute. It is always identical to that interface when it is an
ElementInternals object.ElementInternals object, it
represents a key of the reflected target's target
element's internal content attribute map.ElementInternals.
This means that the ElementInternals interface also has a reflected IDL
attribute, with the same identifier, and that reflected IDL attribute reflects the same reflected content attribute name.fooBar
IDL attribute must reflect the foobar content attribute and support
ElementInternals.ElementInternals object
elementInternals, they are defined as follows:ElementInternals objects as their target
element's internal content attribute map cannot be directly manipulated and as
such reflection is only happening in a single direction. This approach was nevertheless chosen to
make it less error-prone to define IDL attributes that are shared between reflected targets and benefit from common API semantics.
DOMString? that reflect enumerated content attributes can be limited to only known values.
Per the processing models below, those will cause the getters for such IDL attributes to only
return keywords for those enumerated attributes, or the empty string or null.DOMString?:limited to only non-negative numbers and optionally with a default
value defaultValue:
parsedValue.IndexSizeError" this's set the content attribute with the given value
converted to the shortest possible string representing the number as a valid
integer., optionally limited to only positive
numbers, limited to only positive
numbers with fallback, or clamped to the range [clampedMin,
clampedMax], and optionally with a default value defaultValue:
IndexSizeError" minimum be 0.limited to only positive numbers
and optionally with a default value defaultValue:
this and associated attribute's local name is the reflected content
attribute name. Specification authors cannot use support
ElementInternals for IDL attributes of this type.T?,
where T is either Element" for
consistency.FrozenArray<T>?, where T is either
FrozenArray<T>?. It is
initially null.FrozenArray<T>?.element.reflectedElements === element.reflectedElements.Elements" for
consistency.2.6.2 Using reflect in specifications
T?, where T is either T is either
For those, specification authors must use the reflected target's
attr-associated element and attr-associated
elements, respectively. The content attribute presence and value must not be used as they
cannot be fully synchronized with the reflected IDL attribute.2.6.3 Collections
HTMLFormControlsCollection and HTMLOptionsCollection interfaces
are collections derived from the
HTMLAllCollection interface is a collection, but is not so derived.2.6.3.1 The
HTMLAllCollection interfaceHTMLAllCollection interface is used for the legacy document.all attribute. It operates similarly to
HTMLAllCollection objects are rooted at a Document
and have a filter that matches all elements, so the elements represented by the
collection of an HTMLAllCollection object consist of all the descendant
elements of the root Document.HTMLAllCollection interface are legacy platform objects with an additional [[Call]] internal
method described in the section below. They also have an
[[IsHTMLDDA]] internal slot.
HTMLAllCollection interface have several unusual
behaviors, due of the fact that they have an [[IsHTMLDDA]] internal slot:HTMLAllCollection interface.HTMLAllCollection interface, returns true when
compared to the undefined and null values.
(Comparisons using the IsStrictlyEqual abstract
operation, and IsLooselyEqual comparisons to other values such as strings or objects, are
unaffected.)typeof operator in JavaScript returns the string
"undefined" when applied to objects implementing the
HTMLAllCollection interface.document.all as a
way to detect legacy user agents, and one that only supports those legacy user agents and uses
the document.all object without testing for its presence
first. [JAVASCRIPT][
id attributes of all the elements represented by the
collection, and the non-empty values of all the name attributes of
all the "all"-named elements represented by the collection, in
tree order, ignoring later duplicates, with the id of
an element preceding its name if it contributes both, they differ from
each other, and neither is the duplicate of an earlier entry.length getter steps are to return the number
of nodes represented by the collection.namedItem(name) method steps are
to return the result of getting the "all"-named
element(s) from this given name.item(nameOrIndex) method steps
are:
a,
button,
embed,
form,
frame,
frameset,
iframe,
img,
input,
map,
meta,
object,
select, and
textarea
HTMLAllCollection collection given an index index, return the
indexth element in collection, or null if there is no such
indexth element.HTMLAllCollection collection given a name name, perform the
following steps:Document as collection, whose filter matches only elements that are
either:name attribute equal to
name, or,HTMLAllCollection collection given
nameOrIndex:
2.6.3.1.1 [[Call]] ( thisArgument, argumentsList )
result be the result of getting the "all"-indexed or named element(s)
from this HTMLAllCollection given nameOrIndex.Function.prototype.call.call(document.all, null, "x") will still search for
elements. (document.all.call does not exist, since document.all does not inherit from Function.prototype.)2.6.3.2 The
HTMLFormControlsCollection interfaceHTMLFormControlsCollection interface is used for
collections of listed
elements in form elements.Firefox1+Safari4+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet ExplorerNo
Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+Firefox33+Safari7+Chrome21+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?[collection.lengthelement = collection.item(index)element = collection[index]element = collection.namedItem(name)Firefox33+Safari4+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet ExplorerNo
Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+radioNodeList = collection.namedItem(name)element = collection[name]radioNodeList = collection[name]name name from collection.RadioNodeList object containing all
those elements is returned.radioNodeList.valueradioNodeList.value = value
id and name attributes of all the
elements represented by the collection, in tree order, ignoring later
duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is the
duplicate of an earlier entry.namedItem(name) method
must act according to the following algorithm:id attribute or a name
attribute equal to name, then return that node and stop the algorithm.id attribute or a name attribute equal
to name, then return null and stop the algorithm.RadioNodeList object representing a live
view of the HTMLFormControlsCollection object, further filtered so that the only
nodes in the RadioNodeList object are those that have either an id attribute or a name attribute equal
to name. The nodes in the RadioNodeList object must be sorted in
tree order.RadioNodeList object.
RadioNodeList interface inherited from the
Firefox33+Safari7+Chrome21+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?value IDL attribute on the
RadioNodeList object, on getting, must return the value returned by running the
following steps:RadioNodeList object that is an input element whose
type attribute is in the Radio Button state and whose checkedness is true. Otherwise, let it be null.value attribute, return the string "on".value attribute.value IDL attribute must run the
following steps:on": let element be the first element in tree order
represented by the RadioNodeList object that is an input element whose
type attribute is in the Radio Button state and whose value content attribute is either absent, or present and equal to the new value, if any. If no such element exists, then instead let element be null.RadioNodeList object that is an input element whose
type attribute is in the Radio Button state and whose value content attribute is present and equal to the new value, if
any. If no such element exists, then instead let element be null.2.6.3.3 The
HTMLOptionsCollection interfaceFirefox1+Safari3+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer6+
Firefox Android?Safari iOS1+Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+HTMLOptionsCollection interface is used for collections of option elements. It is always
rooted on a select element and has attributes and methods that manipulate that
element's descendants.[collection.lengthcollection.length = valueoption elements in the container corresponding to collection.option elements to the container corresponding to
collection.element = collection.item(index)element = collection[index]collection[index] = elementoption elements in the corresponding container.option element, adds or replaces it at index index in
collection.element = collection.namedItem(name)element = collection[name]name name from collection.collection.add(element[, before])HierarchyRequestError" element is an ancestor of the element into which it is to be inserted.collection.remove(index)collection.selectedIndexcollection.selectedIndex = indexoption element at index index in
collection.
length getter steps are to return the
number of nodes represented by the collection.length setter steps are:length never removes
or adds any optgroup elements, and never adds new children to existing
optgroup elements (though it can remove children from them).id and name attributes of all the
elements represented by the collection, in tree order, ignoring later
duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is
the duplicate of an earlier entry.remove method with index as
the argument, and return.n-1 new option elements with no attributes and
no child nodes to the select element on which the HTMLOptionsCollection
is rooted.select
element. Otherwise, replace the indexth element in the collection by value.add(element, before)
method must act according to the following algorithm:select element on which
the HTMLOptionsCollection is rooted, then throw a
"HierarchyRequestError" before is an element, but that element isn't a descendant of the
select element on which the HTMLOptionsCollection is rooted, then throw
a "NotFoundError" element and before are the same element, then
return.select element on which the HTMLOptionsCollection is rooted.remove(index) method must act
according to the following algorithm:selectedIndex IDL attribute must act
like the identically named attribute on the select element on which the
HTMLOptionsCollection is rooted2.6.4 The
DOMStringList interfaceFirefox1+Safari5.1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android3+Samsung Internet?Opera Android12.1+DOMStringList interface is a non-fashionable retro way of representing a list
of strings.[sequence<DOMString> or
equivalent rather than DOMStringList.strings.lengthstrings[index]strings.item(index)strings.contains(string)DOMStringList object has an associated list.DOMStringList interface supports indexed properties. The
supported property indices are the indices of this's
associated list.Firefox1+Safari5.1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+length getter steps are to return
this's associated list's size.Firefox1+Safari5.1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android3+Samsung Internet?Opera Android12.1+item(index) method steps are to
return the indexth item in this's associated list, or null if
index plus one is greater than this's associated list's size.Firefox1.5+Safari5.1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android3+Samsung Internet?Opera Android12.1+contains(string) method steps
are to return true if this's associated list contains string, and false otherwise.2.7 Safe passing of structured data
structuredClone()
method.)2.7.1 Serializable objects
Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?[Serializable] IDL extended
attribute. Such interfaces must also define the following algorithms:[Serializable] extended attribute must take no
arguments, and must only appear on an interface. It must not appear more than once on an
interface.[Serializable]-annotated interface in the
inheritance chain needs to define standalone serialization steps and
deserialization steps, including taking into account any important data that might
come from inherited interfaces.
Person, which had
associated with it two pieces of associated data:Person instance or
nullPerson instances to be serializable
objects by annotating the Person interface with the [Serializable] extended attribute, and defining the
following accompanying algorithms:2.7.2 Transferable objects
[Transferable] IDL extended
attribute. Such interfaces must also define the following algorithms:[Transferable] extended attribute must take no
arguments, and must only appear on an interface. It must not appear more than once on an
interface.[Transferable]-annotated interface in the
inheritance chain needs to define standalone transfer steps and
transfer-receiving steps, including taking into account any important data that might
come from inherited interfaces.2.7.3 StructuredSerializeInternal ( value, forStorage [ ,
memory ] )
DataCloneError" serialized be an uninitialized value.DataCloneError"
This check is only needed when serializing (and not when deserializing) as
the cross-origin
isolated capability cannot change over time and a SharedArrayBuffer
cannot leave an agent cluster.DataCloneError" value has an [[ArrayBufferMaxByteLength]] internal slot, then set
serialized to { [[Type]]: "GrowableSharedArrayBuffer", [[ArrayBufferData]]:
value.[[ArrayBufferData]], [[ArrayBufferByteLengthData]]:
value.[[ArrayBufferByteLengthData]], [[ArrayBufferMaxByteLength]]:
value.[[ArrayBufferMaxByteLength]], [[AgentCluster]]: the surrounding
agent's agent cluster }.DataCloneError" size be value.[[ArrayBufferByteLength]].RangeError exception
upon allocation failure.DataCloneError" buffer be the value of value's [[ViewedArrayBuffer]] internal
slot.message").stack property, to
serialized.length").DataCloneError" typeString be the identifier of the primary interface of
value.DataCloneError" IsCallable(value) is true, then throw a
"DataCloneError" Otherwise, if value has any internal slot other than [[Prototype]] or
[[Extensible]], then throw a "DataCloneError"
For instance, a [[PromiseState]] or [[WeakMapData]] internal slot.DataCloneError" For instance, a proxy object.
{
[[Type]]: "Object",
[[Properties]]: «
{
[[Key]]: "myself",
[[Value]]: <a pointer to this whole structure>
}
»
}
2.7.4 StructuredSerialize ( value )
2.7.5 StructuredSerializeForStorage ( value )
2.7.6 StructuredDeserialize ( serialized, targetRealm [ ,
memory ] )
ArrayBuffer objects).DataCloneError" value to a new SharedArrayBuffer object in
targetRealm whose [[ArrayBufferData]] internal slot value is
serialized.[[ArrayBufferData]] and whose [[ArrayBufferByteLength]] internal slot
value is serialized.[[ArrayBufferByteLength]].DataCloneError" value to a new SharedArrayBuffer object in
targetRealm whose [[ArrayBufferData]] internal slot value is
serialized.[[ArrayBufferData]], whose [[ArrayBufferByteLengthData]] internal slot
value is serialized.[[ArrayBufferByteLengthData]], and whose
[[ArrayBufferMaxByteLength]] internal slot value is
serialized.[[ArrayBufferMaxByteLength]].DataCloneError" This step might throw an exception if there is not enough memory available to
create such an ArrayBuffer object.DataCloneError" This step might throw an exception if there is not enough memory available to
create such an ArrayBuffer object.message",
messageDesc).DataCloneError" value to a new instance of the interface identified by
interfaceName, created in targetRealm.2.7.7 StructuredSerializeWithTransfer ( value, transferList
)
DataCloneError" transferable has an [[ArrayBufferData]] internal slot and
IsSharedArrayBuffer(transferable) is true, then throw a
"DataCloneError" memory[transferable] exists,
then throw a "DataCloneError" Set memory[transferable] to {
[[Type]]: an uninitialized value }.DataCloneError" transferable has a [[Detached]] internal slot and
transferable.[[Detached]] is true, then throw a
"DataCloneError" dataHolder be memory[transferable].WebAssembly JavaScript Interface, for example. [WASMJS]2.7.8 StructuredDeserializeWithTransfer ( serializeWithTransferResult,
targetRealm )
DataCloneError"
value to a new instance of the interface identified by
interfaceName, created in targetRealm.2.7.9 Performing serialization and
transferring from other specifications
messagePort.postMessage()
uses this pair of abstract operations, as the destination realm is not known until the
MessagePort has been shipped.SharedArrayBuffer objects, since storing shared memory
does not make sense. Similarly, it can throw or possibly have different behavior when given a
platform object with custom serialization steps when the
forStorage argument is true.history.pushState() and history.replaceState() use
StructuredSerializeForStorage on author-supplied state objects, storing them as
serialized state in the appropriate session history entry. Then,
StructuredDeserialize is used so that the history.state property can return a clone of the
originally-supplied state object.broadcastChannel.postMessage() uses
StructuredSerialize on its input, then uses StructuredDeserialize
multiple times on the result to produce a fresh clone for each destination being broadcast
to. Note that transferring does not make sense in multi-destination situations.
window.postMessage() performs
StructuredSerializeWithTransfer on its arguments, but is careful to do so
immediately, inside the synchronous portion of its algorithm. Thus it is able to use the
algorithms without needing to prepare to run script and prepare to run a
callback.2.7.10 Structured cloning API
result = self.structuredClone(value[, { transfer }])transfer array are transferred, not
just cloned, meaning that they are no longer usable in the input value.DataCloneError" serializable.Firefox94+Safari15.4+Chrome98+
Opera?Edge98+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?structuredClone(value,
options) method steps are:transfer"]).3 Semantics, structure, and APIs of HTML documents
3.1 Documents
Document object.
[DOM]Document object's DOM. It is initially set when
the Document object is created, but can change during the lifetime of the
Document object; for example, it changes when the user navigates to a fragment on the page
and when the pushState() method is called with a new
URL. [DOM]Document object's
URL in their user interface. This is the primary
mechanism by which a user can tell if a site is attempting to impersonate another.Document object's DOM. It is initially set when the
Document object is created, and can change during the lifetime of the
Document only upon setting document.domain. A Document's origin can differ from the origin of its URL;
for example when a child navigable is created, its active document's origin is inherited from its parent's active document's origin, even though its active document's URL is
about:blank. [DOM]Document is created by a script using
the or methods, the
Document is ready for post-load tasks immediately.Document is created. If it is not explicitly set, then its value
is the empty string.3.1.1 The
Document objectFirefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android10.1+
Document has a policy container (a policy container), initially a new policy
container, which contains policies which apply to the Document.Document has a permissions policy, which
is a permissions policy, which is initially
empty.Document has a module map,
which is a module map, initially empty.Document has a cross-origin opener
policy, which is a cross-origin opener policy, initially a new cross-origin
opener policy.Document has an is initial about:blank, which is a
boolean, initially false.Document has a during-loading
navigation ID for WebDriver BiDi, which is a navigation ID or null, initially
null.Document's lifecycle, in a way that ties them to the original
navigation ID used when the navigation that created this Document was
the ongoing navigation. This eventually gets set back to null, after WebDriver
BiDi considers the loading process to be finished. [BIDI]Document has an about base
URL, which is a URL or null, initially null.about:"-schemed
Documents.Document has a fire
mutation events flag, which is a boolean, initially true.3.1.2 The
DocumentOrShadowRoot interface
3.1.3 Resource metadata management
document.referrerFirefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+Document from
which the user navigated to this one, unless it was blocked or there was no such document, in
which case it returns the empty string.noreferrer link type can be used to block the
referrer.referrer
attribute must return the document's referrer.
document.cookie [ = value ]Document. If there are no cookies or
cookies can't be applied to this resource, the empty string will be returned.iframe with the sandbox attribute), a
"SecurityError"
Firefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+cookie
attribute represents the cookies of the resource identified by the document's URL.Document object that falls into one of the following conditions is a
cookie-averse Document object:Document object whose browsing
context is null.Document whose URL's scheme is not an HTTP(S) scheme.
On getting, if the document is a cookie-averse Document object, then the
user agent must return the empty string. Otherwise, if the Document's origin is an opaque
origin, the user agent must throw a "SecurityError"
cookie-string
for the document's URL for a "non-HTTP" API, decoded
using UTF-8 decode without BOM. [COOKIES]Document object, then
the user agent must do nothing. Otherwise, if the Document's origin is an opaque
origin, the user agent must throw a "SecurityError"
receiving a set-cookie-string for the document's
URL via a "non-HTTP" API, consisting of the new value
encoded as UTF-8. [COOKIES] [ENCODING]cookie attribute is accessible
across frames, the path restrictions on cookies are only a tool to help manage which cookies are
sent to which parts of the site, and are not in any way a security feature.cookie attribute's getter and
setter synchronously access shared state. Since there is no locking mechanism, other browsing
contexts in a multiprocess user agent can modify cookies while scripts are running. A site could,
for instance, try to read a cookie, increment its value, then write it back out, using the new
value of the cookie as a unique identifier for the session; if the site does this twice in two
different browser windows at the same time, it might end up using the same "unique" identifier for
both sessions, with potentially disastrous effects.
document.lastModified