HTML

When an algorithm B says to return to another algorithm A, it implies that A called B. Upon returning to A, the implementation must continue from where it left off in calling B.

A user agent must not consider the types text/plain and application/octet-stream as having a registered plugin.

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.

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

To eat an orange, the user must:
1. Peel the orange.
2. Separate each slice of the orange.
3. Eat the orange slices.

To eat an orange:
1. The user must peel the orange.
2. The user must separate each slice of the orange.
3. The user must eat the orange slices.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

2.2.1 Conformance classes

This specification describes the conformance criteria for user agents (relevant to implementors) and documents (relevant to authors and authoring tool implementors).

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 agents

Web browsers that support the XHTML 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 XHTML processor would, upon finding an XHTML 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.

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 the Web IDL specification. [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 script element in the example above would still implement the HTMLScriptElement interface.

Non-interactive presentation user agents

User agents that process HTML and XHTML 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 rendering

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 in the rendering section that that section defines as the behavior that user agents are expected to implement.

User agents with no scripting support

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

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 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).

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 tools

Applications and tools that process HTML and XHTML 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 each section would not be conforming.

Authoring tools and markup generators

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 address element for arbitrary contact information; that element can only be used for marking up contact information for the author of the document or section. 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 a section, 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 div, b, i, and span and making liberal use of the style 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.

User agents may impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations.

For compatibility with existing content and prior specifications, this specification describes two authoring formats: one based on XML (referred to as the XHTML syntax), 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.)

2.2.2 Dependencies

This specification relies on several other underlying specifications.

Unicode and Encoding

The Unicode character set is used to represent textual data, and the WHATWG Encoding standard 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 the Encoding specification: [ENCODING]

• Getting an encoding
• The encoder and decoder algorithms for various encodings, including the UTF-8 encoder and UTF-8 decoder
• 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 decoder is distinct from the UTF-8 decode algorithm. The latter first strips a Byte Order Mark (BOM), if any, and then invokes the former.

XML

Implementations that support the XHTML syntax must support some version of XML, as well as its corresponding namespaces specification, because that syntax uses an XML serialization with namespaces. [XML] [XMLNS]

URLs

The following terms are defined in the URL standard: [URL]

• URL
• Absolute URL
• Relative URL
• Relative schemes
• The URL parser
• Parsed URL
• The scheme component of a parsed URL
• The scheme data component of a parsed URL
• The username component of a parsed URL
• The password component of a parsed URL
• The host component of a parsed URL
• The port component of a parsed URL
• The path component of a parsed URL
• The query component of a parsed URL
• The fragment component of a parsed URL
• Parse errors from the URL parser
• The URL serializer
• Default encode set
• Percent encode
• UTF-8 percent encode
• Percent decode
• Decoder error
• URLUtils interface
• URLUtilsReadOnly interface
• href attribute
• protocol attribute
• The get the base hook for URLUtils
• The update steps hook for URLUtils
• The set the input algorithm for URLUtils
• The query encoding of an URLUtils object
• The input of an URLUtils object
• The url of an URLUtils object

Cookies

The following terms are defined in the Cookie specification: [COOKIES]

• cookie-string
• receives a set-cookie-string

CORS

The following terms are defined in the CORS specification: [CORS]

• cross-origin request
• cross-origin request status
• custom request headers
• simple cross-origin request
• redirect steps
• omit credentials flag
• resource sharing check

Web IDL

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]

The terms supported property indices, determine the value of an indexed property, support named properties, supported property names, determine the value of a named property, platform array objects, and read only (when applied to arrays) are used as defined in the Web IDL specification. The algorithm to convert a DOMString to a sequence of Unicode characters is similarly that defined in the Web IDL specification.

Where this specification says an interface or exception is exposed to JavaScript, it refers to the manner, described in the Web IDL specification, in which an ECMAScript global environment exposes interfaces and exceptions.

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 Date object must represent the time after that truncation.

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 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).

JavaScript

Some parts of the language described by this specification only support JavaScript as the underlying scripting language. [ECMA262]

The term "JavaScript" is used to refer to ECMA262, rather than the official term ECMAScript, since the term JavaScript is more widely known. Similarly, the MIME type used to refer to JavaScript in this specification is text/javascript, since that is the most commonly used type, despite it being an officially obsoleted type according to RFC 4329. [RFC4329]

The term JavaScript global environment refers to the global environment concept defined in the ECMAScript specification.

The ECMAScript SyntaxError exception is also defined in the ECMAScript specification. [ECMA262]

DOM

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 DOM 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] [DOMEVENTS]

In particular, the following features are defined in the DOM specification: [DOM]

• Attr interface
• Comment interface
• DOMImplementation interface
• Document interface
• DocumentFragment interface
• DocumentType interface
• DOMException interface
• Element interface
• Node interface
• NodeList interface
• ProcessingInstruction interface
• Text interface
• HTMLCollection interface
• item() method
• The terms collections and represented by the collection
• DOMTokenList interface
• DOMSettableTokenList interface
• createDocument() method
• createHTMLDocument() method
• createElement() method
• createElementNS() method
• getElementById() method
• insertBefore() method
• ownerDocument attribute
• childNodes attribute
• localName attribute
• parentNode attribute
• namespaceURI attribute
• tagName attribute
• id attribute
• textContent attribute
• The insert, append, remove, and replace algorithms for nodes
• The nodes are inserted and nodes are removed concepts
• The attribute list concept.
• The data of a text node.
• Event interface
• EventTarget interface
• EventInit dictionary type
• target attribute
• isTrusted attribute
• The type of an event
• The concept of an event listener and the event listeners associated with an EventTarget
• The concept of a regular event parent and a cross-boundary event parent
• The encoding (herein the character encoding) and content type of a Document
• The distinction between XML documents and HTML documents
• The terms quirks mode, limited-quirks mode, and no-quirks mode
• The algorithm to clone a Node, 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 a DOM range, and the terms start, end, and boundary point as applied to ranges.
• MutationObserver interface
• The MutationObserver scripting environment concept
• The invoke MutationObserver objects algorithm

The term throw in this specification is used as defined in the DOM specification. The following DOMException types are defined in the DOM specification: [DOM]

1. IndexSizeError
3. HierarchyRequestError
4. WrongDocumentError
5. InvalidCharacterError
7. NoModificationAllowedError
8. NotFoundError
9. NotSupportedError
11. InvalidStateError
12. SyntaxError
13. InvalidModificationError
14. NamespaceError
15. InvalidAccessError
18. SecurityError
19. NetworkError
20. AbortError
21. URLMismatchError
22. QuotaExceededError
23. TimeoutError
24. InvalidNodeTypeError
25. DataCloneError

For example, to throw a TimeoutError exception, a user agent would construct a DOMException object whose type was the string "TimeoutError" (and whose code was the number 23, for legacy reasons) and actually throw that object as an exception.

The URL associated with a Document, as defined in the DOM specification, is referred to in this specification as the document's address.

The following features are defined in the DOM Events specification: [DOMEVENTS]

• MouseEvent interface
• MouseEventInit dictionary type
• The UIEvent interface's detail attribute
• click 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 the DOM Parsing and Serialization specification: [DOMPARSING]

• innerHTML
• outerHTML

User agents are also encouraged to implement the features described in the HTML Editing APIs and UndoManager and DOM Transaction specifications. [EDITING] [UNDO]

The following parts of the Fullscreen specification are referenced from this specification, in part to define the rendering of dialog elements, and also to define how the Fullscreen API interacts with the sandboxing features in HTML: [FULLSCREEN]

• The top layer concept
• requestFullscreen()
• The fullscreen enabled flag
• The fully exit fullscreen algorithm

Typed Arrays

The ArrayBuffer and ArrayBufferView interfaces and underlying concepts from the Typed Array Specification are used for several features in this specification. The Uint8ClampedArray interface type is specifically used in the definition of the canvas element's 2D API. [TYPEDARRAY]

File API

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

• Blob
• File
• FileList
• Blob.close()
• Blob.type
• The concept of read errors

XMLHttpRequest

This specification references the XMLHttpRequest specification to define how the two specifications interact and to use its ProgressEvent features. The following features and terms are defined in the XMLHttpRequest specification: [XHR]

• Document response entity body
• XMLHttpRequest base URL
• XMLHttpRequest origin
• XMLHttpRequest referrer source
• ProgressEvent
• Fire a progress event named e

Media Queries

Implementations must support the Media Queries language. [MQ]

CSS modules

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.

In particular, some features require that a string be parsed as a CSS <color> value. When parsing a CSS value, user agents are required by the CSS specifications to apply some error handling rules. These apply to this specification also. [CSSCOLOR] [CSS]

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 term CSS element reference identifier is used as defined in the CSS Image Values and Replaced Content specification to define the API that declares identifiers for use with the CSS 'element()' function. [CSSIMAGES]

Similarly, the term provides a paint source is used as defined in the CSS Image Values and Replaced Content specification to define the interaction of certain HTML elements with the CSS 'element()' function. [CSSIMAGES]

The term default object size is also defined in the CSS Image Values and Replaced Content specification. [CSSIMAGES]

Support for the CSS Object Model is required for implementations that support scripting. The following features and terms are defined in the CSSOM specifications: [CSSOM] [CSSOMVIEW]

• Screen
• LinkStyle
• CSSStyleDeclaration
• cssText attribute of CSSStyleDeclaration
• StyleSheet
• sheet
• disabled
• Alternative style sheet sets and the preferred style sheet set
• Serializing a CSS value
• Scroll an element into view
• Scroll to the beginning of the document

The term CSS styling attribute is defined in the CSS Style Attributes specification. [CSSATTR]

The CanvasRenderingContext2D object's use of fonts depends on the features described in the CSS Fonts specification, including in particular FontLoader. [CSSFONTS]

SVG

The following interface is defined in the SVG specification: [SVG]

• SVGMatrix

WebGL

The following interface is defined in the WebGL specification: [WEBGL]

• WebGLRenderingContext

WebVTT

Implementations may support WebVTT as a text track format for subtitles, captions, chapter titles, metadata, etc, for media resources. [WEBVTT]

The following terms, used in this specification, are defined in the WebVTT specification:

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

The WebSocket protocol

The following terms are defined in the WebSocket protocol specification: [WSP]

• establish a WebSocket connection
• the WebSocket connection is established
• validate the server's response
• extensions in use
• subprotocol in use
• headers to send appropriate cookies
• cookies set during the server's opening handshake
• a WebSocket message has been received
• fail the WebSocket connection
• close the WebSocket connection
• start the WebSocket closing handshake
• the WebSocket closing handshake is started
• the WebSocket connection is closed (possibly cleanly)
• the WebSocket connection close code
• the WebSocket connection close reason

ARIA

The terms strong native semantics is used as defined in the ARIA specification. The term default implicit ARIA semantics has the same meaning as the term implicit WAI-ARIA semantics as used in the ARIA specification. [ARIA]

The role and aria-* attributes are defined in the ARIA specification. [ARIA]

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 the Web Origin Concept specification and the HTTP State Management Mechanism specification (Cookies) as well. [HTTP] [ORIGIN] [COOKIES]

This specification might have certain additional requirements on character encodings, image formats, audio formats, and video formats in the respective sections.

If such extensions are nonetheless needed, e.g. for experimental purposes, then vendors are strongly urged to use one of the following extension mechanisms:

For markup-level features that can be limited to the XML serialization and need not be supported in the HTML serialization, vendors should use the namespace mechanism to define custom namespaces in which the non-standard elements and attributes are supported.

For markup-level features that are intended for use with the HTML syntax, extensions should be limited to new attributes of the form "x-vendor-feature", where vendor is a short string that identifies the vendor responsible for the extension, and feature is the name of the feature. New element names should not be created. Using attributes for such extensions exclusively allows extensions from multiple vendors to co-exist on the same element, which would not be possible with elements. Using the "x-vendor-feature" form allows extensions to be made without risk of conflicting with future additions to the specification.

<p>This smells of lemons!
<span x-ferret-smellovision x-ferret-smellcode="LEM01"
      x-mb-outputsmell x-mb-smell="lemon juice"></span></p>

Attribute names beginning with the two characters "x-" are reserved for user agent use and are guaranteed to never be formally added to the HTML language. For flexibility, attributes names containing underscores (the U+005F LOW LINE character) are also reserved for experimental purposes and are guaranteed to never be formally added to the HTML language.

Pages that use such attributes are by definition non-conforming.

For DOM extensions, e.g. new methods and IDL attributes, the new members should be prefixed by vendor-specific strings to prevent clashes with future versions of this specification.

For events, experimental event types should be prefixed with vendor-specific strings.

All extensions must be defined so that the use of extensions neither contradicts nor causes the non-conformance of functionality defined in the specification.

When adding new reflecting IDL attributes corresponding to content attributes of the form "x-vendor-feature", the IDL attribute should be named "vendorFeature" (i.e. the "x" is dropped from the IDL attribute's name).

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.

Converting a string to ASCII uppercase means replacing all characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z) with the corresponding characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z).

Converting a string to ASCII lowercase means replacing all characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) with the corresponding characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z).