2 Common infrastructure

This specification depends on Infra. [INFRA]

2.1 Terminology

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 <a href="https://html.spec.whatwg.org/multipage/dom.html#document">Document</a> objects and their descendant DOM trees, and to serialized byte streams using the HTML syntax or the XML syntax, depending on context.

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 <a href="https://html.spec.whatwg.org/multipage/dom.html#document">Document</a> objects can find themselves in. [DOM] (Such uses are always hyperlinked to their definition.)

In the context of byte streams, the term HTML document refers to resources labeled as <a href="https://html.spec.whatwg.org/multipage/iana.html#text/html">text/html</a>, and the term XML document refers to resources labeled with an XML MIME type.

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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.

2.1.1 Parallelism

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:

1. Let parallelQueue be a new parallel queue.
2. Run the following steps in parallel:

   1. While true:

      1. Let steps be the result of dequeueing from parallelQueue's algorithm queue.
      2. If steps is not nothing, then run steps.
      3. 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.

3. 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:

1. Let p be a new promise created in this's relevant realm.
2. Run the following steps in parallel:
   1. If nameList contains name, then queue a global task on the DOM manipulation task source given this's relevant global object to reject p with a <a data-x-internal="typeerror" href="https://tc39.es/ecma262/#sec-native-error-types-used-in-this-standard-typeerror">TypeError</a>, and abort these steps.
   2. Do some potentially lengthy work.
   3. Append name to nameList.
   4. Queue a global task on the DOM manipulation task source given this's relevant global object to resolve p with undefined.
3. 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:

1. Let p be a new promise created in this's relevant realm.
2. =Enqueue the following steps to nameListQueue:=
   1. If nameList contains name, then queue a global task on the DOM manipulation task source given this's relevant global object to reject p with a <a data-x-internal="typeerror" href="https://tc39.es/ecma262/#sec-native-error-types-used-in-this-standard-typeerror">TypeError</a>, and abort these steps.
   2. Do some potentially lengthy work.
   3. Append name to nameList.
   4. Queue a global task on the DOM manipulation task source given this's relevant global object to resolve p with undefined.
3. Return p.

The steps would now queue and the race is avoided.

2.1.2 Resources

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 @import rules, including those indirectly imported by other imported style sheets.

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.

2.1.3 XML compatibility

To ease migration from HTML to XML, user agents conforming to this specification will place elements in HTML in the <a data-x-internal="html-namespace-2" href="https://infra.spec.whatwg.org/#html-namespace">http://www.w3.org/1999/xhtml</a> namespace, at least for the purposes of the DOM and CSS. The term "HTML elements" refers to any element in that namespace, even in XML documents.

Except where otherwise stated, all elements defined or mentioned in this specification are in the HTML namespace ("http://www.w3.org/1999/xhtml"), and all attributes defined or mentioned in this specification have no namespace.

The term element type is used to refer to the set of elements that have a given local name and namespace. For example, <a href="https://html.spec.whatwg.org/multipage/form-elements.html#the-button-element">button</a> elements are elements with the element type <a href="https://html.spec.whatwg.org/multipage/form-elements.html#the-button-element">button</a>, meaning they have the local name "button" and (implicitly as defined above) the HTML namespace.

Attribute names are said to be XML-compatible if they match the <a data-x-internal="xml-name" href="https://www.w3.org/TR/xml/#NT-Name">Name</a> production defined in XML and they contain no U+003A COLON characters (😃. [XML]

2.1.4 DOM trees

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, <a data-x-internal="text" href="https://dom.spec.whatwg.org/#interface-text">Text</a> node, or string, means that the length of the text is zero (i.e., not even containing controls or U+0020 SPACE).

An HTML element can have specific HTML element insertion steps, HTML element post-connection steps, and HTML element removing steps, all defined for the element's local name.

The insertion steps for the HTML Standard, given insertedNode, are defined as the following:

1. 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.
2. If insertedNode is a form-associated element or the ancestor of a form-associated element, then:
   1. If the form-associated element's parser inserted flag is set, then return.
   2. Reset the form owner of the form-associated element.
3. If insertedNode is an <a data-x-internal="element" href="https://dom.spec.whatwg.org/#interface-element">Element</a> that is not on the stack of open elements of an HTML parser, then process internal resource links given insertedNode's node document.

The post-connection steps for the HTML Standard, given insertedNode, are defined as the following:

1. If insertedNode is an element whose namespace is the HTML namespace, and this standard defines HTML element post-connection steps for insertedNode's local name, then run the corresponding HTML element post-connection steps given insertedNode.

The removing steps for the HTML Standard, given removedNode and oldParent, are defined as the following:

1. Let document be removedNode's node document.
2. 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 <a href="https://html.spec.whatwg.org/multipage/indices.html#event-blur">blur</a> or <a href="https://html.spec.whatwg.org/multipage/indices.html#event-change">change</a> events are fired.
3. 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.
4. If removedNode is a form-associated element or the ancestor of a form-associated element, then:
   1. 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.
5. If removedNode's <a href="https://html.spec.whatwg.org/multipage/popover.html#attr-popover">popover</a> attribute is not in the no popover state, then run the hide popover algorithm given removedNode, false, false, and false.

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.

2.1.5 Scripting

The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface Foo".

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.

2.1.6 Plugins

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 <a href="https://html.spec.whatwg.org/multipage/dom.html#document">Document</a> object, but that neither act as child navigables of the <a href="https://html.spec.whatwg.org/multipage/dom.html#document">Document</a> nor introduce any <a data-x-internal="node" href="https://dom.spec.whatwg.org/#interface-node">Node</a> objects to the <a href="https://html.spec.whatwg.org/multipage/dom.html#document">Document</a>'s DOM.

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 <a data-x-internal="text/plain" href="https://www.rfc-editor.org/rfc/rfc2046#section-4.1.3">text/plain</a> and <a data-x-internal="application/octet-stream" href="https://www.rfc-editor.org/rfc/rfc2046#section-4.5.1">application/octet-stream</a> as having a registered plugin.

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.

(https://infra.spec.whatwg.org/#tracking-vector "There is a tracking vector here.")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

A character encoding, 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 encoding name and one or more encoding labels, referred to as the encoding's name and labels in the Encoding standard. [ENCODING]

2.1.8 Conformance classes

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 foobar element", it would imply that documents are not allowed to contain elements named foobar.

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 and other interactive user agentsWeb 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 <a href="https://html.spec.whatwg.org/multipage/scripting.html#the-script-element">script</a> 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 <a href="https://html.spec.whatwg.org/multipage/scripting.html#the-script-element">script</a> element as an opaque element that forms part of the transform.

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 <a href="https://html.spec.whatwg.org/multipage/scripting.html#the-script-element">script</a> element in the example above would still implement the <a href="https://html.spec.whatwg.org/multipage/scripting.html#htmlscriptelement">HTMLScriptElement</a> interface.

Non-interactive presentation user agentsUser 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.

Visual user agents that support the suggested default renderingUser 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.

User agents with no scripting supportImplementations 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 checkersConformance 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 <a href="https://html.spec.whatwg.org/multipage/grouping-content.html#the-blockquote-element">blockquote</a> element is not a quote, conformance checkers running without the input of human judgement do not have to check that <a href="https://html.spec.whatwg.org/multipage/grouping-content.html#the-blockquote-element">blockquote</a> elements only contain quoted material).

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:

1. Criteria that can be expressed in a DTD.
2. Criteria that cannot be expressed by a DTD, but can still be checked by a machine.
3. Criteria that can only be checked by a human.

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.

Data mining toolsApplications 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 generatorsAuthoring 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 <a href="https://html.spec.whatwg.org/multipage/sections.html#the-address-element">address</a> element for arbitrary contact information; that element can only be used for marking up contact information for its nearest <a href="https://html.spec.whatwg.org/multipage/sections.html#the-article-element">article</a> or <a href="https://html.spec.whatwg.org/multipage/sections.html#the-body-element">body</a> 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 <a href="https://html.spec.whatwg.org/multipage/sections.html#the-address-element">address</a> 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 <a href="https://html.spec.whatwg.org/multipage/sections.html#the-article-element">article</a> element, that the user really is doing that and not inserting something else instead.

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 <a href="https://html.spec.whatwg.org/multipage/grouping-content.html#the-div-element">div</a>, <a href="https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-b-element">b</a>, <a href="https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-i-element">i</a>, and <a href="https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-span-element">span</a> and making liberal use of the <a href="https://html.spec.whatwg.org/multipage/dom.html#attr-style">style</a> attribute.

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.)

2.1.9 Dependencies

This specification relies on several other underlying specifications.

InfraThe following terms are defined in Infra: [INFRA]

• The general iteration terms while, continue, and break.
• Assert
• implementation-defined
• tracking vector
• code point and its synonym character
• surrogate
• scalar value
• tuple
• noncharacter
• string, code unit, code unit prefix, code unit less than, starts with, ends with, length, and code point length
• The string equality operations is and identical to
• scalar value string
• convert
• ASCII string
• ASCII tab or newline
• ASCII whitespace
• control
• ASCII digit
• ASCII upper hex digit
• ASCII lower hex digit
• ASCII hex digit
• ASCII upper alpha
• ASCII lower alpha
• ASCII alpha
• ASCII alphanumeric
• isomorphic decode
• isomorphic encode
• ASCII lowercase
• ASCII uppercase
• ASCII case-insensitive
• strip newlines
• normalize newlines
• strip leading and trailing ASCII whitespace
• strip and collapse ASCII whitespace
• split a string on ASCII whitespace
• split a string on commas
• collect a sequence of code points and its associated position variable
• skip ASCII whitespace
• The ordered map data structure and the associated definitions for key, value, empty, entry, exists, getting the value of an entry, setting the value of an entry, removing an entry, clear, getting the keys, getting the values, sorting in descending order, size, and iterate
• The list data structure and the associated definitions for append, extend, prepend, replace, remove, empty, contains, size, indices, is empty, item, iterate, and clone sort in ascending order sort in descending order
• The stack data structure and the associated definitions for push and pop
• The queue data structure and the associated definitions for enqueue and dequeue
• The ordered set data structure and the associated definition for append and union
• The struct specification type and the associated definition for item
• The byte sequence data structure
• The forgiving-base64 encode and forgiving-base64 decode algorithms
• exclusive range
• parse a JSON string to an Infra value
• HTML namespace
• MathML namespace
• SVG namespace
• XLink namespace
• XML namespace
• XMLNS namespace

Unicode and EncodingThe 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]

• Getting an encoding
• Get an output encoding
• The generic decode algorithm which takes a byte stream and an encoding and returns a character stream
• The UTF-8 decode algorithm which takes a byte stream and returns a character stream, additionally stripping one leading UTF-8 Byte Order Mark (BOM), if any
• The UTF-8 decode without BOM algorithm which is identical to UTF-8 decode except that it does not strip one leading UTF-8 Byte Order Mark (BOM)
• The encode algorithm which takes a character stream and an encoding and returns a byte stream
• The UTF-8 encode algorithm which takes a character stream and returns a byte stream
• The BOM sniff algorithm which takes a byte stream and returns an encoding or null.

XML and related specificationsImplementations 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.

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The attribute with the name space in the XML namespace is defined by Extensible Markup Language (XML). [XML]

The Name production is defined in XML. [XML]

This specification also references the <?xml-stylesheet?> processing instruction, defined in Associating Style Sheets with XML documents. [XMLSSPI]

This specification also non-normatively mentions the XSLTProcessor interface and its transformToFragment() and transformToDocument() methods. [XSLTP]

URLsThe following terms are defined in URL: [URL]

• host
• public suffix
• domain
• IP address
• URL
• Origin of URLs
• Absolute URL
• Relative URL
• registrable domain
• The URL parser
• The basic URL parser and its url and state override arguments, as well as these parser states:
  • scheme start state
  • host state
  • hostname state
  • port state
  • path start state
  • query state
  • fragment state
• URL record, as well as its individual components:
  • scheme
  • username
  • password
  • host
  • port
  • path
  • query
  • fragment
  • blob URL entry
• valid URL string
• The cannot have a username/password/port concept
• The opaque path concept
• URL serializer and its exclude fragment argument
• URL path serializer
• The host parser
• The host serializer
• Host equals
• URL equals and its exclude fragments argument
• serialize an integer
• Default encode set
• component percent-encode set
• UTF-8 percent-encode
• percent-decode
• set the username
• set the password
• The application/x-www-form-urlencoded format
• The application/x-www-form-urlencoded serializer
• is special

A number of schemes and protocols are referenced by this specification also:

• The about: scheme [ABOUT]
• The blob: scheme [FILEAPI]
• The data: scheme [RFC2397]
• The http: scheme [HTTP]
• The https: scheme [HTTP]
• The mailto: scheme [MAILTO]
• The sms: scheme [SMS]
• The urn: scheme [URN]

Media fragment syntax is defined in Media Fragments URI. [MEDIAFRAG]

HTTP and related specificationsThe following terms are defined in the HTTP specifications: [HTTP]

• [Accept](https://httpwg.org/specs/rfc7231.html#header.accept) header
• [Accept-Language](https://httpwg.org/specs/rfc7231.html#header.accept-language) header
• [Cache-Control](https://httpwg.org/specs/rfc7234.html#header.cache-control) header
• [Content-Disposition](https://httpwg.org/specs/rfc6266.html) header
• [Content-Language](https://httpwg.org/specs/rfc7231.html#header.content-language) header
• [Content-Range](https://httpwg.org/specs/rfc7233.html#header.content-range) header
• [Last-Modified](https://httpwg.org/specs/rfc7232.html#header.last-modified) header
• [Range](https://httpwg.org/specs/rfc7233.html#header.range) header
• [Referer](https://httpwg.org/specs/rfc7231.html#header.referer) header

The following terms are defined in HTTP State Management Mechanism: [COOKIES]

• cookie-string
• receives a set-cookie-string
• [Cookie](https://httpwg.org/specs/rfc6265.html#cookie) header

The following term is defined in Web Linking: [WEBLINK]

• [Link](https://httpwg.org/specs/rfc8288.html#header) header
• Parsing a Link field value

The following terms are defined in Structured Field Values for HTTP: [STRUCTURED-FIELDS]

• structured header
• boolean
• token
• parameters

The following terms are defined in MIME Sniffing: [MIMESNIFF]

• MIME type
• MIME type essence
• valid MIME type string
• valid MIME type string with no parameters
• HTML MIME type
• JavaScript MIME type and JavaScript MIME type essence match
• JSON MIME type
• XML MIME type
• image MIME type
• audio or video MIME type
• font MIME type
• parse a MIME type
• is MIME type supported by the user agent?

FetchThe following terms are defined in Fetch: [FETCH]

• ABNF
• about:blank
• An HTTP(S) scheme
• A URL which is local
• A local scheme
• A fetch scheme
• CORS protocol
• default User-Agent value
• extract a MIME type
• legacy extract an encoding
• fetch
• fetch controller
• process the next manual redirect
• ok status
• navigation request
• network error
• aborted network error
• [Origin](https://fetch.spec.whatwg.org/#http-origin) header
• [Cross-Origin-Resource-Policy](https://fetch.spec.whatwg.org/#http-cross-origin-resource-policy) header
• getting a structured field value
• header list
• set
• get, decode, and split
• abort
• cross-origin resource policy check
• the RequestCredentials enumeration
• the RequestDestination enumeration
• the fetch() method
• report timing
• serialize a response URL for reporting
• safely extracting a body
• incrementally reading a body
• processResponseConsumeBody
• processResponseEndOfBody
• processResponse
• useParallelQueue
• processEarlyHintsResponse
• connection pool
• obtain a connection
• determine the network partition key
• extract full timing info
• as a body
• response body info
• resolve an origin
• response and its associated:
  • type
  • URL
  • URL list
  • status
  • header list
  • body
  • body info
  • internal response
  • location URL
  • timing info
  • service worker timing info
  • has-cross-origin-redirects
  • timing allow passed
  • extract content-range values
• request and its associated:
  • URL
  • method
  • header list
  • body
  • client
  • URL list
  • current URL
  • reserved client
  • replaces client id
  • initiator
  • destination
  • potential destination
  • translating a potential destination
  • script-like destinations
  • priority
  • origin
  • referrer
  • synchronous flag
  • mode
  • credentials mode
  • use-URL-credentials flag
  • unsafe-request flag
  • cache mode
  • redirect count
  • redirect mode
  • policy container
  • referrer policy
  • cryptographic nonce metadata
  • integrity metadata
  • parser metadata
  • reload-navigation flag
  • history-navigation flag
  • user-activation
  • render-blocking
  • initiator type
  • add a range header
• fetch timing info and its associated:
  • start time
  • end time

The following terms are defined in Referrer Policy: [REFERRERPOLICY]

• referrer policy
• The [Referrer-Policy](https://w3c.github.io/webappsec-referrer-policy/#referrer-policy-header-dfn) HTTP header
• The parse a referrer policy from a Referrer-Policy header algorithm
• The "no-referrer", "no-referrer-when-downgrade", "origin-when-cross-origin", and "unsafe-url" referrer policies
• The default referrer policy

The following terms are defined in Mixed Content: [MIX]

• a priori authenticated URL

The following terms are defined in Subresource Integrity: [SRI]

• parse integrity metadata
• the requirements of the integrity attribute
• get the strongest metadata from set

Paint TimingThe following terms are defined in Paint Timing: [PAINTTIMING]

• mark paint timing

Navigation TimingThe following terms are defined in Navigation Timing: [NAVIGATIONTIMING]

• create the navigation timing entry
• queue the navigation timing entry
• NavigationTimingType and its "navigate", "reload", and "back_forward" values.

Resource TimingThe following terms are defined in Resource Timing: [RESOURCETIMING]

• Mark resource timing

Performance TimelineThe following terms are defined in Performance Timeline: [PERFORMANCETIMELINE]

• PerformanceEntry and its name, entryType, startTime, and duration attributes.
• Queue a performance entry

Long Animation FramesThe following terms are defined in Long Animation Frames: [LONGANIMATIONFRAMES]

• record task start time
• record task end time
• record rendering time
• record classic script creation time
• record classic script execution start time
• record module script execution start time
• Record pause duration
• record timing info for timer handler
• record timing info for microtask checkpoint

Long TasksThe following terms are defined in Long Tasks: [LONGTASKS]

• report long tasks

Web IDLThe 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:

• this
• extended attribute
• named constructor
• constructor operation
• overridden constructor steps
• internally create a new object implementing the interface
• array index property name
• supports indexed properties
• supported property indices
• determine the value of an indexed property
• set the value of an existing indexed property
• set the value of a new indexed property
• support named properties
• supported property names
• determine the value of a named property
• set the value of an existing named property
• set the value of a new named property
• delete an existing named property
• perform a security check
• platform object
• legacy platform object
• primary interface
• interface object
• named properties object
• include
• inherit
• interface prototype object
• implements
• associated realm
• [[Realm]] field of a platform object
• [[GetOwnProperty]] internal method of a named properties object
• callback context
• frozen array and creating a frozen array
• create a new object implementing the interface
• callback this value
• converting between Web IDL types and JS types
• invoking and constructing callback functions
• overload resolution algorithm
• exposed
• a promise resolved with
• a promise rejected with
• wait for all
• upon rejection
• upon fulfillment
• mark as handled
• [Global]
• [LegacyFactoryFunction]
• [LegacyLenientThis]
• [LegacyNullToEmptyString]
• [LegacyOverrideBuiltIns]
• LegacyPlatformObjectGetOwnProperty
• [LegacyTreatNonObjectAsNull]
• [LegacyUnenumerableNamedProperties]
• [LegacyUnforgeable]
• set entries

Web IDL also defines the following types that are used in Web IDL fragments in this specification:

• ArrayBuffer
• ArrayBufferView
• boolean
• DOMString
• double
• enumeration
• Function
• long
• object
• Promise
• Uint8ClampedArray
• unrestricted double
• unsigned long
• USVString
• VoidFunction

The term throw in this specification is used as defined in Web IDL. The DOMException type and the following exception names are defined by Web IDL and used by this specification:

• "IndexSizeError"
• "HierarchyRequestError"
• "InvalidCharacterError"
• "NoModificationAllowedError"
• "NotFoundError"
• "NotSupportedError"
• "InvalidStateError"
• "SyntaxError"
• "InvalidAccessError"
• "SecurityError"
• "NetworkError"
• "AbortError"
• "QuotaExceededError"
• "DataCloneError"
• "EncodingError"
• "NotAllowedError"

When this specification requires a user agent to create a 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 <a data-x-internal="date" href="https://tc39.es/ecma262/#sec-date-objects">Date</a> object must represent the resulting truncated time.

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 <a data-x-internal="date" href="https://tc39.es/ecma262/#sec-date-objects">Date</a> 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 <a data-x-internal="date" href="https://tc39.es/ecma262/#sec-date-objects">Date</a> object that represents a time value NaN (indicating that the object does not represent a specific instant of time).

JavaScriptSome 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:

• active function object
• agent and agent cluster
• automatic semicolon insertion
• candidate execution
• The current realm
• clamping a mathematical value
• early error
• forward progress
• invariants of the essential internal methods
• JavaScript execution context
• JavaScript execution context stack
• realm
• JobCallback Record
• NewTarget
• running JavaScript execution context
• surrounding agent
• abstract closure
• immutable prototype exotic object
• Well-Known Symbols, including %Symbol.hasInstance%, %Symbol.isConcatSpreadable%, %Symbol.toPrimitive%, and %Symbol.toStringTag%
• Well-Known Intrinsic Objects, including %Array.prototype%, %Error.prototype%, %EvalError.prototype%, %Function.prototype%, %JSON.parse%, %Object.prototype%, %Object.prototype.valueOf%, %RangeError.prototype%, %ReferenceError.prototype%, %SyntaxError.prototype%, %TypeError.prototype%, and %URIError.prototype%
• The FunctionBody production
• The Module production
• The Pattern production
• The Script production
• The Type notation
• The Completion Record specification type
• The List and Record specification types
• The Property Descriptor specification type
• The Script Record specification type
• The Cyclic Module Record specification type
• The Source Text Module Record specification type and its Evaluate, Link and LoadRequestedModules methods
• The ArrayCreate abstract operation
• The Call abstract operation
• The ClearKeptObjects abstract operation
• The CleanupFinalizationRegistry abstract operation
• The Construct abstract operation
• The CopyDataBlockBytes abstract operation
• The CreateBuiltinFunction abstract operation
• The CreateByteDataBlock abstract operation
• The CreateDataProperty abstract operation
• The DefinePropertyOrThrow abstract operation
• The DetachArrayBuffer abstract operation
• The EnumerableOwnProperties abstract operation
• The FinishLoadingImportedModule abstract operation
• The OrdinaryFunctionCreate abstract operation
• The Get abstract operation
• The GetActiveScriptOrModule abstract operation
• The GetFunctionRealm abstract operation
• The HasOwnProperty abstract operation
• The HostCallJobCallback abstract operation
• The HostEnqueueFinalizationRegistryCleanupJob abstract operation
• The HostEnqueueGenericJob abstract operation
• The HostEnqueuePromiseJob abstract operation
• The HostEnqueueTimeoutJob abstract operation
• The HostEnsureCanAddPrivateElement abstract operation
• The HostLoadImportedModule abstract operation
• The HostMakeJobCallback abstract operation
• The HostPromiseRejectionTracker abstract operation
• The InitializeHostDefinedRealm abstract operation
• The IsAccessorDescriptor abstract operation
• The IsCallable abstract operation
• The IsConstructor abstract operation
• The IsDataDescriptor abstract operation
• The IsDetachedBuffer abstract operation
• The IsSharedArrayBuffer abstract operation
• The NewObjectEnvironment abstract operation
• The NormalCompletion abstract operation
• The OrdinaryGetPrototypeOf abstract operation
• The OrdinarySetPrototypeOf abstract operation
• The OrdinaryIsExtensible abstract operation
• The OrdinaryPreventExtensions abstract operation
• The OrdinaryGetOwnProperty abstract operation
• The OrdinaryDefineOwnProperty abstract operation
• The OrdinaryGet abstract operation
• The OrdinarySet abstract operation
• The OrdinaryDelete abstract operation
• The OrdinaryOwnPropertyKeys abstract operation
• The OrdinaryObjectCreate abstract operation
• The ParseModule abstract operation
• The ParseScript abstract operation
• The NewPromiseReactionJob abstract operation
• The NewPromiseResolveThenableJob abstract operation
• The RegExpBuiltinExec abstract operation
• The RegExpCreate abstract operation
• The RunJobs abstract operation
• The SameValue abstract operation
• The ScriptEvaluation abstract operation
• The SetImmutablePrototype abstract operation
• The ToBoolean abstract operation
• The ToString abstract operation
• The ToUint32 abstract operation
• The TypedArrayCreate abstract operation
• The IsLooselyEqual abstract operation
• The IsStrictlyEqual abstract operation
• The Atomics object
• The Atomics.waitAsync object
• The Date class
• The FinalizationRegistry class
• The RegExp class
• The SharedArrayBuffer class
• The SyntaxError class
• The TypeError class
• The RangeError class
• The WeakRef class
• The eval() function
• The WeakRef.prototype.deref() function
• The [[IsHTMLDDA]] internal slot
• import()
• import.meta
• The HostGetImportMetaProperties abstract operation
• The typeof operator
• The delete operator
• The TypedArray Constructors table

Users agents that support JavaScript must also implement the Dynamic Code Brand Checks proposal. The following terms are defined there, and used in this specification: [JSDYNAMICCODEBRANDCHECKS]

• The HostEnsureCanCompileStrings abstract operation
• The HostGetCodeForEval abstract operation

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]

• The ModuleRequest Record specification type
• The HostGetSupportedImportAttributes abstract operation

User agents that support JavaScript must also implement the JSON modules proposal. The following terms are defined there, and used in this specification: [JSJSONMODULES]

• The CreateDefaultExportSyntheticModule abstract operation
• The SetSyntheticModuleExport abstract operation
• The Synthetic Module Record specification type
• The ParseJSONModule abstract operation

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 IsArrayBufferViewOutOfBounds abstract operation

User agents that support JavaScript must also implement the Temporal proposal. The following terms are defined there, and used in this specification: [JSTEMPORAL]

• The HostSystemUTCEpochNanoseconds abstract operation
• The nsMaxInstant and nsMinInstant values

WebAssemblyThe following term is defined in WebAssembly JavaScript Interface: [WASMJS]

• WebAssembly.Module

DOMThe 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]

• Attr interface
• CharacterData interface
• Comment interface
• DOMImplementation interface
• Document interface and its doctype attribute
• DocumentOrShadowRoot interface
• DocumentFragment interface
• DocumentType interface
• ChildNode interface
• Element interface
• attachShadow() method.
• An element's shadow root
• A shadow root's mode
• A shadow root's declarative member
• The attach a shadow root algorithm
• The retargeting algorithm
• Node interface
• NodeList interface
• ProcessingInstruction interface
• ShadowRoot interface
• Text interface
• Range interface
• node document concept
• document type concept
• host concept
• The shadow root concept, and its delegates focus, available to element internals, clonable, and serializable.
• The shadow host concept
• HTMLCollection interface, its length attribute, and its item() and namedItem() methods
• The terms collection and represented by the collection
• DOMTokenList interface, and its value attribute and supports operation
• createDocument() method
• createHTMLDocument() method
• createElement() method
• createElementNS() method
• getElementById() method
• getElementsByClassName() method
• append() method
• appendChild() method
• cloneNode() method
• importNode() method
• preventDefault() method
• id attribute
• setAttribute() method
• textContent attribute
• The tree, shadow tree, and node tree concepts
• The tree order and shadow-including tree order concepts
• The element concept
• The child concept
• The root and shadow-including root concepts
• The inclusive ancestor, descendant, shadow-including ancestor, shadow-including descendant, shadow-including inclusive descendant, and shadow-including inclusive ancestor concepts
• The first child, next sibling, previous sibling, and parent concepts
• The parent element concept
• The document element concept
• The in a document tree, in a document (legacy), and connected concepts
• The slot concept, and its name and assigned nodes
• The assigned slot concept
• The slot assignment concept
• The slottable concept
• The assign slottables for a tree algorithm
• The slotchange event
• The inclusive descendant concept
• The find flattened slottables algorithm
• The manual slot assignment concept
• The assign a slot algorithm
• The pre-insert, insert, append, replace, replace all, string replace all, remove, and adopt algorithms for nodes
• The descendant concept
• The insertion steps,
• The post-connection steps, removing steps, adopting steps, and children changed steps hooks for elements
• The change, append, remove, replace, get an attribute by namespace and local name, set value, and remove an attribute by namespace and local name algorithms for attributes
• The attribute change steps hook for attributes
• The value concept for attributes
• The local name concept for attributes
• The attribute list concept
• The data of a <a data-x-internal="characterdata" href="https://dom.spec.whatwg.org/#interface-characterdata">CharacterData</a> node and its replace data algorithm
• The child text content of a node
• The descendant text content of a node
• The name, public ID, and system ID of a doctype
• Event interface
• Event and derived interfaces constructor behavior
• EventTarget interface
• The activation behavior hook
• The legacy-pre-activation behavior hook
• The legacy-canceled-activation behavior hook
• The create an event algorithm
• The fire an event algorithm
• The canceled flag
• The dispatch flag
• The dispatch algorithm
• EventInit dictionary type
• type attribute
• An event's target
• currentTarget attribute
• bubbles attribute
• cancelable attribute
• composed attribute
• composed flag
• isTrusted attribute
• initEvent() method
• add an event listener
• addEventListener() method
• The remove an event listener and remove all event listeners algorithms
• EventListener callback interface
• The type of an event
• An event listener and its type and callback
• The encoding (herein the character encoding ), mode, allow declarative shadow roots, and content type of a <a href="https://html.spec.whatwg.org/multipage/dom.html#document">Document</a>
• The distinction between XML documents and HTML documents
• The terms quirks mode, limited-quirks mode, and no-quirks mode
• The algorithm to clone a <a data-x-internal="node" href="https://dom.spec.whatwg.org/#interface-node">Node</a>, and the concept of cloning steps used by that algorithm
• The concept of base URL change steps and the definition of what happens when an element is affected by a base URL change
• The concept of an element's unique identifier (ID)
• The concept of an element's classes
• The term supported tokens
• The concept of a DOM range, and the terms start node, start, end, and boundary point as applied to ranges.
• The create an element algorithm
• The element interface concept
• The concepts of custom element state, and of defined and custom elements
• An element's namespace, namespace prefix, local name, custom element definition, and is value
• MutationObserver interface and mutation observers in general
• AbortController and its signal
• AbortSignal
• aborted
• signal abort
• add
• The get an attribute by name algorithm

The following features are defined in UI Events: [UIEVENTS]

• The MouseEvent interface
• The <a data-x-internal="mouseevent" href="https://w3c.github.io/uievents/#mouseevent">MouseEvent</a> interface's relatedTarget attribute
• MouseEventInit dictionary type
• The FocusEvent interface
• The <a data-x-internal="focusevent" href="https://w3c.github.io/uievents/#focusevent">FocusEvent</a> interface's relatedTarget attribute
• The UIEvent interface
• The <a data-x-internal="uievent" href="https://w3c.github.io/uievents/#uievent">UIEvent</a> interface's view attribute
• auxclick event
• beforeinput event
• click event
• contextmenu event
• dblclick event
• input event
• mousedown event
• mouseenter event
• mouseleave event
• mousemove event
• mouseout event
• mouseover event
• mouseup event
• wheel event
• keydown event
• keypress event
• keyup event

The following features are defined in Touch Events: [TOUCH]

• Touch interface
• Touch point concept
• touchend event

The following features are defined in Pointer Events: [POINTEREVENTS]

• The PointerEvent interface
• The <a data-x-internal="pointerevent" href="https://w3c.github.io/pointerevents/#pointerevent-interface">PointerEvent</a> interface's pointerType attribute
• fire a pointer event
• pointerdown event
• pointerup event
• pointercancel event

The following events are defined in Clipboard API and events: [CLIPBOARD-APIS]

• copy event
• cut event
• paste event

This specification sometimes uses the term name to refer to the event's type; as in, "an event named click" or "if the event name is keypress". The terms "name" and "type" for events are synonymous.

The following features are defined in DOM Parsing and Serialization: [DOMPARSING]

• XML serialization

The following features are defined in Selection API: [SELECTION]

• selection
• Selection

User agents are encouraged to implement the features described in execCommand. [EXECCOMMAND]

The following features are defined in Fullscreen API: [FULLSCREEN]

• requestFullscreen()
• fullscreenchange
• run the fullscreen steps
• fully exit fullscreen
• fullscreen element
• fullscreen flag

High Resolution Time provides the following features: [HRT]

• current high resolution time
• relative high resolution time
• unsafe shared current time
• shared monotonic clock
• unsafe moment
• duration from
• coarsen time
• current wall time
• Unix epoch
• DOMHighResTimeStamp

File APIThis specification uses the following features defined in File API: [FILEAPI]

• The Blob interface and its type attribute
• The File interface and its name and lastModified attributes
• The FileList interface
• The concept of a <a data-x-internal="blob" href="https://w3c.github.io/FileAPI/#dfn-Blob">Blob</a>'s snapshot state
• The concept of read errors
• Blob URL Store
• blob URL entry and its object and environment

Indexed Database APIThis specification uses cleanup Indexed Database transactions defined by Indexed Database API. [INDEXEDDB]

Media Source ExtensionsThe following terms are defined in Media Source Extensions: [MEDIASOURCE]

• MediaSource interface
• detaching from a media element

Media Capture and StreamsThe following terms are defined in Media Capture and Streams: [MEDIASTREAM]

• MediaStream interface

ReportingThe following terms are defined in Reporting: [REPORTING]

• Queue a report
• report type
• visible to ReportingObservers

XMLHttpRequestThe following features and terms are defined in XMLHttpRequest: [XHR]

• The XMLHttpRequest interface, and its responseXML attribute
• The ProgressEvent interface, and its lengthComputable, loaded, and total attributes
• The FormData interface, and its associated entry list

Battery StatusThe following features are defined in Battery Status API: [BATTERY]

• getBattery() method

Media QueriesImplementations must support Media Queries. The <media-condition> feature is defined therein. [MQ]

CSS modulesWhile 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 "rgb(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.

The following terms and features are defined in Cascading Style Sheets (CSS): [CSS]

• viewport
• line box
• out-of-flow
• in-flow
• collapsing margins
• containing block
• inline box
• block box
• The 'top', 'bottom', 'left', and 'right' properties
• The 'float' property
• The 'clear' property
• The 'width' property
• The 'height' property
• The 'min-width' property
• The 'min-height' property
• The 'max-width' property
• The 'max-height' property
• The 'line-height' property
• The 'vertical-align' property
• The 'content' property
• The 'inline-block' value of the 'display' property
• The 'visibility' property

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]

• content area
• content box
• border box
• margin box
• border edge
• margin edge
• The 'margin-top', 'margin-bottom', 'margin-left', and 'margin-right' properties
• The 'padding-top', 'padding-bottom', 'padding-left', and 'padding-right' properties

The following features are defined in CSS Logical Properties: [CSSLOGICAL]

• The 'margin-block', 'margin-block-start', 'margin-block-end', 'margin-inline', 'margin-inline-start', and 'margin-inline-end' properties
• The 'padding-block', 'padding-block-start', 'padding-block-end', 'padding-inline', 'padding-inline-start', and 'padding-inline-end' properties
• The 'border-block-width', 'border-block-start-width', 'border-block-end-width', 'border-inline-width', 'border-inline-start-width', 'border-inline-end-width', 'border-block-style', 'border-block-start-style', 'border-block-end-style', 'border-inline-style', 'border-inline-start-style', 'border-inline-end-style', 'border-block-start-color', 'border-block-end-color', 'border-inline-start-color', 'border-inline-end-color', 'border-start-start-radius', 'border-start-end-radius', 'border-end-start-radius', and 'border-end-end-radius' properties
• The 'block-size' property
• The 'inline-size' property
• The 'inset-block-start' property
• The 'inset-block-end' property

The following terms and features are defined in CSS Color: [CSSCOLOR]

• named color
• <color>
• The 'color' property
• The 'currentcolor' value
• opaque black
• transparent black
• 'srgb' color space
• 'display-p3' color space
• 'relative-colorimetric' rendering intent
• parse a CSS <color> value

The following terms are defined in CSS Images: [CSSIMAGES]

• default object size
• concrete object size
• natural dimensions
• natural height
• natural width
• The 'image-orientation' property
• 'conic-gradient'
• The 'object-fit' property

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]

• The 'background-color', 'background-image', 'background-repeat', 'background-attachment', 'background-position', 'background-clip', 'background-origin', and 'background-size' properties
• The 'border-radius', 'border-top-left-radius', 'border-top-right-radius', 'border-bottom-right-radius', 'border-bottom-left-radius' properties
• The 'border-image-source', 'border-image-slice', 'border-image-width', 'border-image-outset', and 'border-image-repeat' properties

CSS Backgrounds and Borders also defines the following border properties: [CSSBG]

Border properties| | 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' |

The following features are defined in CSS Box Alignment: [CSSALIGN]

• The 'align-content' property
• The 'align-items' property
• The 'align-self' property
• The 'justify-self' property
• The 'justify-content' property
• The 'justify-items' property

The following terms and features are defined in CSS Display: [CSSDISPLAY]

• outer display type
• inner display type
• block-level
• block container
• formatting context
• block formatting context
• inline formatting context
• replaced element
• CSS box

The following features are defined in CSS Flexible Box Layout: [CSSFLEXBOX]

• The 'flex-direction' property
• The 'flex-wrap' property

The following terms and features are defined in CSS Fonts: [CSSFONTS]

• first available font
• The 'font-family' property
• The 'font-weight' property
• The 'font-size' property
• The 'font' property
• The 'font-kerning' property
• The 'font-stretch' property
• The 'font-variant-caps' property
• The 'small-caps' value
• The 'all-small-caps' value
• The 'petite-caps' value
• The 'all-petite-caps' value
• The 'unicase' value
• The 'titling-caps' value
• The 'ultra-condensed' value
• The 'extra-condensed' value
• The 'condensed' value
• The 'semi-condensed' value
• The 'semi-expanded' value
• The 'expanded' value
• The 'extra-expanded' value
• The 'ultra-expanded' value

The following features are defined in CSS Grid Layout: [CSSGRID]

• The 'grid-auto-columns' property
• The 'grid-auto-flow' property
• The 'grid-auto-rows' property
• The 'grid-column-gap' property
• The 'grid-row-gap' property
• The 'grid-template-areas' property
• The 'grid-template-columns' property
• The 'grid-template-rows' property

The following terms are defined in CSS Inline Layout: [CSSINLINE]

• alphabetic baseline
• ascent metric
• descent metric
• hanging baseline
• ideographic-under baseline

The following terms and features are defined in CSS Box Sizing: [CSSSIZING]

• fit-content inline size
• 'aspect-ratio' property
• intrinsic size

The following features are defined in CSS Lists and Counters. [CSSLISTS]

• list item
• The 'counter-reset' property
• The 'counter-set' property
• The 'list-style-type' property

The following features are defined in CSS Overflow. [CSSOVERFLOW]

• The 'overflow' property and its 'hidden' value
• The 'text-overflow' property
• The term scroll container

The following terms and features are defined in CSS Positioned Layout: [CSSPOSITION]

• absolutely-positioned
• The 'position' property and its 'static' value
• The top layer (an ordered set)
• add an element to the top layer
• request an element to be removed from the top layer
• remove an element from the top layer immediately
• process top layer removals

The following features are defined in CSS Multi-column Layout. [CSSMULTICOL]

• The 'column-count' property
• The 'column-fill' property
• The 'column-gap' property
• The 'column-rule' property
• The 'column-width' property

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 'border-spacing' property
• The 'border-collapse' property
• The 'table-cell', 'table-row', 'table-caption', and 'table' values of the 'display' property

The following features are defined in CSS Text: [CSSTEXT]

• The 'text-transform' property
• The 'white-space' property
• The 'text-align' property
• The 'letter-spacing' property
• The 'word-spacing' property

The following features are defined in CSS Writing Modes: [CSSWM]

• The 'direction' property
• The 'unicode-bidi' property
• The 'writing-mode' property
• The block flow direction, block axis, inline axis, block size, inline size, block-start, block-end, inline-start, inline-end, line-left, and line-right concepts

The following features are defined in CSS Basic User Interface: [CSSUI]

• The 'outline' property
• The 'cursor' property
• The 'appearance' property, its <compat-auto> non-terminal value type, its 'textfield' value, and its 'menulist-button' value.
• The 'field-sizing' property, and its 'content' value.
• The concept widget
• The concept native appearance
• The concept primitive appearance
• The concept element with default preferred size
• The non-devolvable widget and devolvable widget classification, and the related devolved widget state.
• The 'pointer-events' property
• The 'user-select' property

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]

• Screen interface
• LinkStyle interface
• CSSStyleDeclaration interface
• style IDL attribute
• cssText attribute of <a data-x-internal="cssstyledeclaration" href="https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface">CSSStyleDeclaration</a>
• StyleSheet interface
• CSSStyleSheet interface
• create a CSS style sheet
• remove a CSS style sheet
• associated CSS style sheet
• create a constructed CSSStyleSheet
• synchronously replace the rules of a CSSStyleSheet
• disable a CSS style sheet
• CSS style sheets and their properties:
  • type
  • location
  • parent CSS style sheet
  • owner node
  • owner CSS rule
  • media
  • title
  • alternate flag
  • disabled flag
  • CSS rules
  • origin-clean flag
• CSS style sheet set
• CSS style sheet set name
• preferred CSS style sheet set name
• change the preferred CSS style sheet set name
• Serializing a CSS value
• run the resize steps
• run the scroll steps
• evaluate media queries and report changes
• Scroll a target into view
• Scroll to the beginning of the document
• The resize event
• The scroll event
• The scrollend event
• set up browsing context features

The following features and terms are defined in CSS Syntax: [CSSSYNTAX]

• conformant style sheet
• parse a list of component values
• parse a comma-separated list of component values
• component value
• environment encoding
• <whitespace-token>

The following terms are defined in Selectors: [SELECTORS]

• type selector
• attribute selector
• pseudo-class
• :focus-visible pseudo-class
• indicate focus
• pseudo-element

The following features are defined in CSS Values and Units: [CSSVALUES]

• <length>
• The 'em' unit
• The 'ex' unit
• The 'vw' unit
• The 'in' unit
• The 'px' unit
• The 'pt' unit
• The 'attr()' function
• The math functions

The following features are defined in CSS View Transitions: [CSSVIEWTRANSITIONS]

• perform pending transition operations
• rendering suppression for view transitions
• activate view transition
• ViewTransition
• view transition page visibility change steps
• resolving inbound cross-document view-transition
• setting up a cross-document view-transition
• can navigation trigger a cross-document view-transition?

The term style attribute is defined in CSS Style Attributes. [CSSATTR]

The following terms are defined in the CSS Cascading and Inheritance: [CSSCASCADE]

• cascaded value
• specified value
• computed value
• used value
• cascade origin
• Author Origin
• User Origin
• User Agent Origin
• Animation Origin
• Transition Origin
• initial value

The <a href="https://html.spec.whatwg.org/multipage/canvas.html#canvasrenderingcontext2d">CanvasRenderingContext2D</a> 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]

The following interfaces and terms are defined in Geometry Interfaces: [GEOMETRY]

• DOMMatrix interface, and associated m11 element, m12 element, m21 element, m22 element, m41 element, and m42 element
• DOMMatrix2DInit and DOMMatrixInit dictionaries
• The create a DOMMatrix from a dictionary and create a DOMMatrix from a 2D dictionary algorithms for <a data-x-internal="dommatrix2dinit" href="https://drafts.fxtf.org/geometry/#dictdef-dommatrix2dinit">DOMMatrix2DInit</a> or <a data-x-internal="dommatrixinit" href="https://drafts.fxtf.org/geometry/#dictdef-dommatrixinit">DOMMatrixInit</a>
• The DOMPointInit dictionary, and associated x and y members
• Matrix multiplication

The following terms are defined in the CSS Scoping: [CSSSCOPING]

• flat tree

The following terms and features are defined in CSS Color Adjustment: [CSSCOLORADJUST]

• 'color-scheme'
• page's supported color-schemes

The following terms are defined in CSS Pseudo-Elements: [CSSPSEUDO]

• '::details-content'
• '::file-selector-button'

The following terms are defined in CSS Containment: [CSSCONTAIN]

• skips its contents
• relevant to the user
• proximity to the viewport
• layout containment
• 'content-visibility' property
• 'auto' value for 'content-visibility'

Intersection ObserverThe following term is defined in Intersection Observer: [INTERSECTIONOBSERVER]

• run the update intersection observations steps
• IntersectionObserver
• IntersectionObserverInit
• observe
• unobserve
• isIntersecting
• target

Resize ObserverThe following terms are defined in Resize Observer: [RESIZEOBSERVER]

• gather active resize observations at depth
• has active resize observations
• has skipped resize observations
• broadcast active resize observations
• deliver resize loop error

WebGLThe following interfaces are defined in the WebGL specifications: [WEBGL]

• WebGLRenderingContext interface
• WebGL2RenderingContext interface
• WebGLContextAttributes dictionary

WebGPUThe following interfaces are defined in WebGPU: [WEBGPU]

• GPUCanvasContext interface

WebVTTImplementations 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:

• WebVTT file
• WebVTT file using cue text
• WebVTT file using only nested cues
• WebVTT parser
• The rules for updating the display of WebVTT text tracks
• The WebVTT text track cue writing direction
• VTTCue interface

ARIAThe role attribute is defined in Accessible Rich Internet Applications (ARIA), as are the following roles: [ARIA]

• button
• presentation

In addition, the following aria-* content attributes are defined in ARIA: [ARIA]

• aria-checked
• aria-describedby
• aria-disabled
• aria-label

Finally, the following terms are defined ARIA: [ARIA]

• role
• accessible name
• The ARIAMixin interface, with its associated ARIAMixin getter steps and ARIAMixin setter steps hooks

Content Security PolicyThe following terms are defined in Content Security Policy: [CSP]

• Content Security Policy
• disposition
• directive set
• Content Security Policy directive
• CSP list
• The Content Security Policy syntax
• enforce the policy
• The parse a serialized Content Security Policy algorithm
• The Run CSP initialization for a Document algorithm
• The Run CSP initialization for a global object algorithm
• The Should element's inline behavior be blocked by Content Security Policy? algorithm
• The Should navigation request of type be blocked by Content Security Policy? algorithm
• The Should navigation response to navigation request of type in target be blocked by Content Security Policy? algorithm
• The report-uri directive
• The EnsureCSPDoesNotBlockStringCompilation abstract operation
• The Is base allowed for Document? algorithm
• The frame-ancestors directive
• The sandbox directive
• The contains a header-delivered Content Security Policy property.
• The Parse a response's Content Security Policies algorithm.
• SecurityPolicyViolationEvent interface
• The securitypolicyviolation event

Service WorkersThe following terms are defined in Service Workers: [SW]

• active worker
• client message queue
• control
• handle fetch
• match service worker registration
• service worker
• service worker client
• ServiceWorker interface
• ServiceWorkerContainer interface
• ServiceWorkerGlobalScope interface

Secure ContextsThe following algorithms are defined in Secure Contexts: [SECURE-CONTEXTS]

• Is url potentially trustworthy?

Permissions PolicyThe following terms are defined in Permissions Policy: [PERMISSIONSPOLICY]

• permissions policy
• policy-controlled feature
• container policy
• serialized permissions policy
• default allowlist
• The creating a permissions policy algorithm
• The creating a permissions policy from a response algorithm
• The is feature enabled by policy for origin algorithm
• The process permissions policy attributes algorithm

Payment Request APIThe following feature is defined in Payment Request API: [PAYMENTREQUEST]

• PaymentRequest interface

MathMLWhile 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):

• MathML annotation-xml element
• MathML math element
• MathML merror element
• MathML mi element
• MathML mn element
• MathML mo element
• MathML ms element
• MathML mtext element

SVGWhile 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]

• SVGElement interface
• SVGImageElement interface
• SVGScriptElement interface
• SVGSVGElement interface
• SVG a element
• SVG desc element
• SVG foreignObject element
• SVG image element
• SVG script element
• SVG svg element
• SVG title element
• SVG use element
• SVG text-rendering property

Filter EffectsThe following features are defined in Filter Effects: [FILTERS]

• <filter-value-list>

CompositingThe following features are defined in Compositing and Blending: [COMPOSITE]

• <blend-mode>
• <composite-mode>
• source-over
• copy

Cooperative Scheduling of Background TasksThe following features are defined in Cooperative Scheduling of Background Tasks: [REQUESTIDLECALLBACK]

• requestIdleCallback()
• start an idle period algorithm

Screen OrientationThe following terms are defined in Screen Orientation: [SCREENORIENTATION]

• screen orientation change steps

StorageThe following terms are defined in Storage: [STORAGE]

• obtain a local storage bottle map
• obtain a session storage bottle map
• obtain a storage key for non-storage purposes
• storage key equal
• storage proxy map
• legacy-clone a traversable storage shed

Web App ManifestThe following features are defined in Web App Manifest: [MANIFEST]

• application manifest
• installed web application
• process the manifest

WebAssembly JavaScript Interface: ESM IntegrationThe following terms are defined in WebAssembly JavaScript Interface: ESM Integration: [WASMESM]

• WebAssembly Module Record
• parse a WebAssembly module

WebCodecsThe following features are defined in WebCodecs: [WEBCODECS]

• VideoFrame interface.
• [[display width]]
• [[display height]]

WebDriverThe following terms are defined in WebDriver: [WEBDRIVER]

• extension command
• remote end steps
• WebDriver error
• WebDriver error code
• invalid argument
• getting a property
• success
• WebDriver's security considerations
• current browsing context

WebDriver BiDiThe following terms are defined in WebDriver BiDi: [WEBDRIVERBIDI]

• WebDriver BiDi navigation status
• navigation status id
• navigation status status
• navigation status canceled
• navigation status pending
• navigation status complete
• navigation status url
• WebDriver BiDi navigation started
• WebDriver BiDi navigation aborted
• WebDriver BiDi navigation failed
• WebDriver BiDi download started
• WebDriver BiDi fragment navigated
• WebDriver BiDi DOM content loaded
• WebDriver BiDi load complete
• WebDriver BiDi navigable created
• WebDriver BiDi navigable destroyed
• WebDriver BiDi user prompt closed
• WebDriver BiDi user prompt opened

Web Cryptography APIThe following terms are defined in Web Cryptography API: [WEBCRYPTO]

• generating a random UUID

WebSocketsThe following terms are defined in WebSockets: [WEBSOCKETS]

• WebSocket
• make disappear

WebTransportThe following terms are defined in WebTransport: [WEBTRANSPORT]

• WebTransport
• context cleanup steps

Web Authentication: An API for accessing Public Key CredentialsThe following terms are defined in Web Authentication: An API for accessing Public Key Credentials: [WEBAUTHN]

• public key credential

Credential ManagementThe following terms are defined in Credential Management: [CREDMAN]

• conditional mediation
• credential
• navigator.credentials.get()

ConsoleThe following terms are defined in Console: [CONSOLE]

• report a warning to the console

Web Locks APIThe following terms are defined in Web Locks API: [WEBLOCKS]

• locks
• lock requests

Trusted TypesThis specification uses the following features defined in Trusted Types: [TRUSTED-TYPES]

• TrustedHTML
• data
• TrustedScript
• data
• TrustedScriptURL
• Get Trusted Type compliant string

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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.

2.1.10 Extensibility

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 "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 <a href="https://html.spec.whatwg.org/multipage/forms.html#dom-form-elements">elements</a> array would be in violation of the above requirement, as it would violate the definition of <a href="https://html.spec.whatwg.org/multipage/forms.html#dom-form-elements">elements</a> given in this specification.

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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.

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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.

2.1.11 Interactions with XPath and XSLT

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 document.evaluate() API) must act as if the following edit was applied to the XPath 1.0 specification.

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 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.

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:

1. If the context node is from an HTML DOM, the default element namespace is "http://www.w3.org/1999/xhtml".
2. Otherwise, the default element namespace URI is null.

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]

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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]

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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 "interactive" and then to "complete" if they are aborted.

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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 script element section, and of XSLT, XPath, and HTML in the template element section.