Table of contents

Full table of contents

1 Introduction

1.1 Where does this specification fit?

This specification defines a big part of the web platform, in lots of detail. Its place in the web platform specification stack relative to other specifications can be best summed up as follows:

CSS SVG MathML Service Workers IDB Fetch CSP AV1 Opus PNG THIS SPECIFICATION HTTP TLS DOM Unicode Web IDL MIME URL XML JavaScript Encoding

1.2 Is this HTML5?

This section is non-normative.

In short: Yes.

In more length: the term "HTML5" is widely used as a buzzword to refer to modern web technologies, many of which (though by no means all) are developed at the WHATWG. This document is one such; others are available from the WHATWG Standards overview.

1.3 Background

This section is non-normative.

HTML is the World Wide Web's core markup language. Originally, HTML was primarily designed as a language for semantically describing scientific documents. Its general design, however, has enabled it to be adapted, over the subsequent years, to describe a number of other types of documents and even applications.

1.4 Audience

This section is non-normative.

This specification is intended for authors of documents and scripts that use the features defined in this specification, implementers of tools that operate on pages that use the features defined in this specification, and individuals wishing to establish the correctness of documents or implementations with respect to the requirements of this specification.

This document is probably not suited to readers who do not already have at least a passing familiarity with web technologies, as in places it sacrifices clarity for precision, and brevity for completeness. More approachable tutorials and authoring guides can provide a gentler introduction to the topic.

In particular, familiarity with the basics of DOM is necessary for a complete understanding of some of the more technical parts of this specification. An understanding of Web IDL, HTTP, XML, Unicode, character encodings, JavaScript, and CSS will also be helpful in places but is not essential.

1.5 Scope

This section is non-normative.

This specification is limited to providing a semantic-level markup language and associated semantic-level scripting APIs for authoring accessible pages on the web ranging from static documents to dynamic applications.

The scope of this specification does not include providing mechanisms for media-specific customization of presentation (although default rendering rules for web browsers are included at the end of this specification, and several mechanisms for hooking into CSS are provided as part of the language).

The scope of this specification is not to describe an entire operating system. In particular, hardware configuration software, image manipulation tools, and applications that users would be expected to use with high-end workstations on a daily basis are out of scope. In terms of applications, this specification is targeted specifically at applications that would be expected to be used by users on an occasional basis, or regularly but from disparate locations, with low CPU requirements. Examples of such applications include online purchasing systems, searching systems, games (especially multiplayer online games), public telephone books or address books, communications software (email clients, instant messaging clients, discussion software), document editing software, etc.

1.6 History

This section is non-normative.

For its first five years (1990-1995), HTML went through a number of revisions and experienced a number of extensions, primarily hosted first at CERN, and then at the IETF.

With the creation of the W3C, HTML's development changed venue again. A first abortive attempt at extending HTML in 1995 known as HTML 3.0 then made way to a more pragmatic approach known as HTML 3.2, which was completed in 1997. HTML4 quickly followed later that same year.

The following year, the W3C membership decided to stop evolving HTML and instead begin work on an XML-based equivalent, called XHTML. This effort started with a reformulation of HTML4 in XML, known as XHTML 1.0, which added no new features except the new serialization, and which was completed in 2000. After XHTML 1.0, the W3C's focus turned to making it easier for other working groups to extend XHTML, under the banner of XHTML Modularization. In parallel with this, the W3C also worked on a new language that was not compatible with the earlier HTML and XHTML languages, calling it XHTML2.

Around the time that HTML's evolution was stopped in 1998, parts of the API for HTML developed by browser vendors were specified and published under the name DOM Level 1 (in 1998) and DOM Level 2 Core and DOM Level 2 HTML (starting in 2000 and culminating in 2003). These efforts then petered out, with some DOM Level 3 specifications published in 2004 but the working group being closed before all the Level 3 drafts were completed.

In 2003, the publication of XForms, a technology which was positioned as the next generation of web forms, sparked a renewed interest in evolving HTML itself, rather than finding replacements for it. This interest was borne from the realization that XML's deployment as a web technology was limited to entirely new technologies (like RSS and later Atom), rather than as a replacement for existing deployed technologies (like HTML).

A proof of concept to show that it was possible to extend HTML4's forms to provide many of the features that XForms 1.0 introduced, without requiring browsers to implement rendering engines that were incompatible with existing HTML web pages, was the first result of this renewed interest. At this early stage, while the draft was already publicly available, and input was already being solicited from all sources, the specification was only under Opera Software's copyright.

The idea that HTML's evolution should be reopened was tested at a W3C workshop in 2004, where some of the principles that underlie the HTML5 work (described below), as well as the aforementioned early draft proposal covering just forms-related features, were presented to the W3C jointly by Mozilla and Opera. The proposal was rejected on the grounds that the proposal conflicted with the previously chosen direction for the web's evolution; the W3C staff and membership voted to continue developing XML-based replacements instead.

Shortly thereafter, Apple, Mozilla, and Opera jointly announced their intent to continue working on the effort under the umbrella of a new venue called the WHATWG. A public mailing list was created, and the draft was moved to the WHATWG site. The copyright was subsequently amended to be jointly owned by all three vendors, and to allow reuse of the specification.

The WHATWG was based on several core principles, in particular that technologies need to be backwards compatible, that specifications and implementations need to match even if this means changing the specification rather than the implementations, and that specifications need to be detailed enough that implementations can achieve complete interoperability without reverse-engineering each other.

The latter requirement in particular required that the scope of the HTML5 specification include what had previously been specified in three separate documents: HTML4, XHTML1, and DOM2 HTML. It also meant including significantly more detail than had previously been considered the norm.

In 2006, the W3C indicated an interest to participate in the development of HTML5 after all, and in 2007 formed a working group chartered to work with the WHATWG on the development of the HTML5 specification. Apple, Mozilla, and Opera allowed the W3C to publish the specification under the W3C copyright, while keeping a version with the less restrictive license on the WHATWG site.

For a number of years, both groups then worked together. In 2011, however, the groups came to the conclusion that they had different goals: the W3C wanted to publish a "finished" version of "HTML5", while the WHATWG wanted to continue working on a Living Standard for HTML, continuously maintaining the specification rather than freezing it in a state with known problems, and adding new features as needed to evolve the platform.

In 2019, the WHATWG and W3C signed an agreement to collaborate on a single version of HTML going forward: this document.

1.7 Design notes

This section is non-normative.

It must be admitted that many aspects of HTML appear at first glance to be nonsensical and inconsistent.

HTML, its supporting DOM APIs, as well as many of its supporting technologies, have been developed over a period of several decades by a wide array of people with different priorities who, in many cases, did not know of each other's existence.

Features have thus arisen from many sources, and have not always been designed in especially consistent ways. Furthermore, because of the unique characteristics of the web, implementation bugs have often become de-facto, and now de-jure, standards, as content is often unintentionally written in ways that rely on them before they can be fixed.

Despite all this, efforts have been made to adhere to certain design goals. These are described in the next few subsections.

1.7.1 Serializability of script execution

This section is non-normative.

To avoid exposing web authors to the complexities of multithreading, the HTML and DOM APIs are designed such that no script can ever detect the simultaneous execution of other scripts. Even with workers, the intent is that the behavior of implementations can be thought of as completely serializing the execution of all scripts in all browsing contexts.

The exception to this general design principle is the JavaScript SharedArrayBuffer class. Using SharedArrayBuffer objects, it can in fact be observed that scripts in other agents are executing simultaneously. Furthermore, due to the JavaScript memory model, there are situations which not only are un-representable via serialized script execution, but also un-representable via serialized statement execution among those scripts.

1.7.2 Compliance with other specifications

This section is non-normative.

This specification interacts with and relies on a wide variety of other specifications. In certain circumstances, unfortunately, conflicting needs have led to this specification violating the requirements of these other specifications. Whenever this has occurred, the transgressions have each been noted as a " willful violation ", and the reason for the violation has been noted.

1.7.3 Extensibility

This section is non-normative.

HTML has a wide array of extensibility mechanisms that can be used for adding semantics in a safe manner:

Authors can use the class attribute to extend elements, effectively creating their own elements, while using the most applicable existing "real" HTML element, so that browsers and other tools that don't know of the extension can still support it somewhat well. This is the tack used by microformats, for example.

Authors can include data for inline client-side scripts or server-side site-wide scripts to process using the data-*="" attributes. These are guaranteed to never be touched by browsers, and allow scripts to include data on HTML elements that scripts can then look for and process.

Authors can use the <meta name="" content=""> mechanism to include page-wide metadata.

Authors can use the rel="" mechanism to annotate links with specific meanings by registering extensions to the predefined set of link types. This is also used by microformats.

Authors can embed raw data using the <script type=""> mechanism with a custom type, for further handling by inline or server-side scripts.

Authors can create plugins and invoke them using the embed element. This is how Flash works.

Authors can extend APIs using the JavaScript prototyping mechanism. This is widely used by script libraries, for instance.

Authors can use the microdata feature (the itemscope="" and itemprop="" attributes) to embed nested name-value pairs of data to be shared with other applications and sites.

1.8 HTML vs XML syntax

This section is non-normative.

This specification defines an abstract language for describing documents and applications, and some APIs for interacting with in-memory representations of resources that use this language.

The in-memory representation is known as "DOM HTML", or "the DOM" for short.

There are various concrete syntaxes that can be used to transmit resources that use this abstract language, two of which are defined in this specification.

The first such concrete syntax is the HTML syntax. This is the format suggested for most authors. It is compatible with most legacy web browsers. If a document is transmitted with the text/html MIME type, then it will be processed as an HTML document by web browsers. This specification defines the latest HTML syntax, known simply as "HTML".

The second concrete syntax is XML. When a document is transmitted with an XML MIME type, such as application/xhtml+xml , then it is treated as an XML document by web browsers, to be parsed by an XML processor. Authors are reminded that the processing for XML and HTML differs; in particular, even minor syntax errors will prevent a document labeled as XML from being rendered fully, whereas they would be ignored in the HTML syntax.

The XML syntax for HTML was formerly referred to as "XHTML", but this specification does not use that term (among other reasons, because no such term is used for the HTML syntaxes of MathML and SVG).

The DOM, the HTML syntax, and the XML syntax cannot all represent the same content. For example, namespaces cannot be represented using the HTML syntax, but they are supported in the DOM and in the XML syntax. Similarly, documents that use the noscript feature can be represented using the HTML syntax, but cannot be represented with the DOM or in the XML syntax. Comments that contain the string " --> " can only be represented in the DOM, not in the HTML and XML syntaxes.

1.9 Structure of this specification

This section is non-normative.

This specification is divided into the following major sections:

Introduction Non-normative materials providing a context for the HTML standard. Common infrastructure The conformance classes, algorithms, definitions, and the common underpinnings of the rest of the specification. Semantics, structure, and APIs of HTML documents Documents are built from elements. These elements form a tree using the DOM. This section defines the features of this DOM, as well as introducing the features common to all elements, and the concepts used in defining elements. The elements of HTML Each element has a predefined meaning, which is explained in this section. Rules for authors on how to use the element, along with user agent requirements for how to handle each element, are also given. This includes large signature features of HTML such as video playback and subtitles, form controls and form submission, and a 2D graphics API known as the HTML canvas. Microdata This specification introduces a mechanism for adding machine-readable annotations to documents, so that tools can extract trees of name-value pairs from the document. This section describes this mechanism and some algorithms that can be used to convert HTML documents into other formats. This section also defines some sample Microdata vocabularies for contact information, calendar events, and licensing works. User interaction HTML documents can provide a number of mechanisms for users to interact with and modify content, which are described in this section, such as how focus works, and drag-and-drop. Loading web pages HTML documents do not exist in a vacuum — this section defines many of the features that affect environments that deal with multiple pages, such as web browsers and offline caching of web applications. Web application APIs This section introduces basic features for scripting of applications in HTML. Web workers This section defines an API for background threads in JavaScript. The communication APIs This section describes some mechanisms that applications written in HTML can use to communicate with other applications from different domains running on the same client. It also introduces a server-push event stream mechanism known as Server Sent Events or EventSource , and a two-way full-duplex socket protocol for scripts known as Web Sockets. Web storage This section defines a client-side storage mechanism based on name-value pairs. The HTML syntax The XML syntax All of these features would be for naught if they couldn't be represented in a serialized form and sent to other people, and so these sections define the syntaxes of HTML and XML, along with rules for how to parse content using those syntaxes. Rendering This section defines the default rendering rules for web browsers.

There are also some appendices, listing obsolete features and IANA considerations, and several indices.

1.9.1 How to read this specification

This specification should be read like all other specifications. First, it should be read cover-to-cover, multiple times. Then, it should be read backwards at least once. Then it should be read by picking random sections from the contents list and following all the cross-references.

As described in the conformance requirements section below, this specification describes conformance criteria for a variety of conformance classes. In particular, there are conformance requirements that apply to producers, for example authors and the documents they create, and there are conformance requirements that apply to consumers, for example web browsers. They can be distinguished by what they are requiring: a requirement on a producer states what is allowed, while a requirement on a consumer states how software is to act.

For example, "the foo attribute's value must be a valid integer" is a requirement on producers, as it lays out the allowed values; in contrast, the requirement "the foo attribute's value must be parsed using the rules for parsing integers" is a requirement on consumers, as it describes how to process the content.

Requirements on producers have no bearing whatsoever on consumers.

Continuing the above example, a requirement stating that a particular attribute's value is constrained to being a valid integer emphatically does not imply anything about the requirements on consumers. It might be that the consumers are in fact required to treat the attribute as an opaque string, completely unaffected by whether the value conforms to the requirements or not. It might be (as in the previous example) that the consumers are required to parse the value using specific rules that define how invalid (non-numeric in this case) values are to be processed.

1.9.2 Typographic conventions

This is a definition, requirement, or explanation.

This is a note.

This is an example.

This is an open issue.

This is a warning.

[ Exposed = Window ] interface Example { // this is an IDL definition };

variable = object . method ( [ optionalArgument ] ) This is a note to authors describing the usage of an interface.

/* this is a CSS fragment */

The defining instance of a term is marked up like this . Uses of that term are marked up like this or like this.

The defining instance of an element, attribute, or API is marked up like this . References to that element, attribute, or API are marked up like this .

Other code fragments are marked up like this .

Variables are marked up like this .

In an algorithm, steps in synchronous sections are marked with ⌛.

In some cases, requirements are given in the form of lists with conditions and corresponding requirements. In such cases, the requirements that apply to a condition are always the first set of requirements that follow the condition, even in the case of there being multiple sets of conditions for those requirements. Such cases are presented as follows:

This is a condition This is another condition This is the requirement that applies to the conditions above. This is a third condition This is the requirement that applies to the third condition.

1.10 A quick introduction to HTML

This section is non-normative.

A basic HTML document looks like this:

<!DOCTYPE html> < html lang = "en" > < head > < title > Sample page </ title > </ head > < body > < h1 > Sample page </ h1 > < p > This is a < a href = "demo.html" > simple </ a > sample. </ p > <!-- this is a comment --> </ body > </ html >

HTML documents consist of a tree of elements and text. Each element is denoted in the source by a start tag, such as " <body> ", and an end tag, such as " </body> ". (Certain start tags and end tags can in certain cases be omitted and are implied by other tags.)

Tags have to be nested such that elements are all completely within each other, without overlapping:

< p > This is < em > very < strong > wrong </ em > ! </ strong ></ p >

< p > This < em > is < strong > correct </ strong > . </ em ></ p >

This specification defines a set of elements that can be used in HTML, along with rules about the ways in which the elements can be nested.

Elements can have attributes, which control how the elements work. In the example below, there is a hyperlink, formed using the a element and its href attribute:

< a href = "demo.html" > simple </ a >

Attributes are placed inside the start tag, and consist of a name and a value, separated by an " = " character. The attribute value can remain unquoted if it doesn't contain ASCII whitespace or any of " ' ` = < or > . Otherwise, it has to be quoted using either single or double quotes. The value, along with the " = " character, can be omitted altogether if the value is the empty string.

<!-- empty attributes --> < input name = address disabled > < input name = address disabled = "" > <!-- attributes with a value --> < input name = address maxlength = 200 > < input name = address maxlength = '200' > < input name = address maxlength = "200" >

HTML user agents (e.g., web browsers) then parse this markup, turning it into a DOM (Document Object Model) tree. A DOM tree is an in-memory representation of a document.

DOM trees contain several kinds of nodes, in particular a DocumentType node, Element nodes, Text nodes, nodes, and in some cases ProcessingInstruction nodes.

The markup snippet at the top of this section would be turned into the following DOM tree:

DOCTYPE: html

html lang =" en " =" head #text : ⏎␣␣ title #text : Sample page #text : ⏎␣ #text : ⏎␣ body #text : ⏎␣␣ h1 #text : Sample page #text : ⏎␣␣ p #text : This is a a href =" demo.html " =" #text : simple #text : sample. #text : ⏎␣␣ : this is a comment #text : ⏎␣⏎



The document element of this tree is the html element, which is the element always found in that position in HTML documents. It contains two elements, head and body , as well as a Text node between them.

There are many more Text nodes in the DOM tree than one would initially expect, because the source contains a number of spaces (represented here by "␣") and line breaks ("⏎") that all end up as Text nodes in the DOM. However, for historical reasons not all of the spaces and line breaks in the original markup appear in the DOM. In particular, all the whitespace before head start tag ends up being dropped silently, and all the whitespace after the body end tag ends up placed at the end of the body .

The head element contains a title element, which itself contains a Text node with the text "Sample page". Similarly, the body element contains an h1 element, a p element, and a comment.

This DOM tree can be manipulated from scripts in the page. Scripts (typically in JavaScript) are small programs that can be embedded using the script element or using event handler content attributes. For example, here is a form with a script that sets the value of the form's output element to say "Hello World":

< form name = "main" > Result: < output name = "result" ></ output > < script > document . forms . main . elements . result . value = 'Hello World' ; </ script > </ form >

Each element in the DOM tree is represented by an object, and these objects have APIs so that they can be manipulated. For instance, a link (e.g. the a element in the tree above) can have its " href " attribute changed in several ways:

var a = document . [ 0 ]; // obtain the first link in the document a . href = 'sample.html' ; // change the destination URL of the link a . protocol = 'https' ; // change just the scheme part of the URL a . setAttribute ( 'href' , 'https://example.com/' ); // change the content attribute directly

Since DOM trees are used as the way to represent HTML documents when they are processed and presented by implementations (especially interactive implementations like web browsers), this specification is mostly phrased in terms of DOM trees, instead of the markup described above.

HTML documents represent a media-independent description of interactive content. HTML documents might be rendered to a screen, or through a speech synthesizer, or on a braille display. To influence exactly how such rendering takes place, authors can use a styling language such as CSS.

In the following example, the page has been made yellow-on-blue using CSS.

<!DOCTYPE html> < html lang = "en" > < head > < title > Sample styled page </ title > < style > body { background : navy ; color : yellow ; } </ style > </ head > < body > < h1 > Sample styled page </ h1 > < p > This page is just a demo. </ p > </ body > </ html >

For more details on how to use HTML, authors are encouraged to consult tutorials and guides. Some of the examples included in this specification might also be of use, but the novice author is cautioned that this specification, by necessity, defines the language with a level of detail that might be difficult to understand at first.

1.10.1 Writing secure applications with HTML

This section is non-normative.

When HTML is used to create interactive sites, care needs to be taken to avoid introducing vulnerabilities through which attackers can compromise the integrity of the site itself or of the site's users.

A comprehensive study of this matter is beyond the scope of this document, and authors are strongly encouraged to study the matter in more detail. However, this section attempts to provide a quick introduction to some common pitfalls in HTML application development.

The security model of the web is based on the concept of "origins", and correspondingly many of the potential attacks on the web involve cross-origin actions. [ORIGIN]

Not validating user input Cross-site scripting (XSS) SQL injection When accepting untrusted input, e.g. user-generated content such as text comments, values in URL parameters, messages from third-party sites, etc, it is imperative that the data be validated before use, and properly escaped when displayed. Failing to do this can allow a hostile user to perform a variety of attacks, ranging from the potentially benign, such as providing bogus user information like a negative age, to the serious, such as running scripts every time a user looks at a page that includes the information, potentially propagating the attack in the process, to the catastrophic, such as deleting all data in the server. When writing filters to validate user input, it is imperative that filters always be safelist-based, allowing known-safe constructs and disallowing all other input. Blocklist-based filters that disallow known-bad inputs and allow everything else are not secure, as not everything that is bad is yet known (for example, because it might be invented in the future). For example, suppose a page looked at its URL's query string to determine what to display, and the site then redirected the user to that page to display a message, as in: < ul > < li >< a href = "message.cgi?say=Hello" > Say Hello </ a > < li >< a href = "message.cgi?say=Welcome" > Say Welcome </ a > < li >< a href = "message.cgi?say=Kittens" > Say Kittens </ a > </ ul > If the message was just displayed to the user without escaping, a hostile attacker could then craft a URL that contained a script element: https://example.com/message.cgi?say=%3Cscript%3Ealert%28%27Oh%20no%21%27%29%3C/script%3E If the attacker then convinced a victim user to visit this page, a script of the attacker's choosing would run on the page. Such a script could do any number of hostile actions, limited only by what the site offers: if the site is an e-commerce shop, for instance, such a script could cause the user to unknowingly make arbitrarily many unwanted purchases. This is called a cross-site scripting attack. There are many constructs that can be used to try to trick a site into executing code. Here are some that authors are encouraged to consider when writing safelist filters: When allowing harmless-seeming elements like img , it is important to safelist any provided attributes as well. If one allowed all attributes then an attacker could, for instance, use the onload attribute to run arbitrary script.

, it is important to safelist any provided attributes as well. If one allowed all attributes then an attacker could, for instance, use the attribute to run arbitrary script. When allowing URLs to be provided (e.g. for links), the scheme of each URL also needs to be explicitly safelisted, as there are many schemes that can be abused. The most prominent example is " javascript: ", but user agents can implement (and indeed, have historically implemented) others.

", but user agents can implement (and indeed, have historically implemented) others. Allowing a base element to be inserted means any script elements in the page with relative links can be hijacked, and similarly that any form submissions can get redirected to a hostile site. Cross-site request forgery (CSRF) If a site allows a user to make form submissions with user-specific side-effects, for example posting messages on a forum under the user's name, making purchases, or applying for a passport, it is important to verify that the request was made by the user intentionally, rather than by another site tricking the user into making the request unknowingly. This problem exists because HTML forms can be submitted to other origins. Sites can prevent such attacks by populating forms with user-specific hidden tokens, or by checking ` Origin ` headers on all requests. Clickjacking A page that provides users with an interface to perform actions that the user might not wish to perform needs to be designed so as to avoid the possibility that users can be tricked into activating the interface. One way that a user could be so tricked is if a hostile site places the victim site in a small iframe and then convinces the user to click, for instance by having the user play a reaction game. Once the user is playing the game, the hostile site can quickly position the iframe under the mouse cursor just as the user is about to click, thus tricking the user into clicking the victim site's interface. To avoid this, sites that do not expect to be used in frames are encouraged to only enable their interface if they detect that they are not in a frame (e.g. by comparing the window object to the value of the top attribute).

1.10.2 Common pitfalls to avoid when using the scripting APIs

This section is non-normative.

Scripts in HTML have "run-to-completion" semantics, meaning that the browser will generally run the script uninterrupted before doing anything else, such as firing further events or continuing to parse the document.

On the other hand, parsing of HTML files happens incrementally, meaning that the parser can pause at any point to let scripts run. This is generally a good thing, but it does mean that authors need to be careful to avoid hooking event handlers after the events could have possibly fired.

There are two techniques for doing this reliably: use event handler content attributes, or create the element and add the event handlers in the same script. The latter is safe because, as mentioned earlier, scripts are run to completion before further events can fire.

One way this could manifest itself is with img elements and the load event. The event could fire as soon as the element has been parsed, especially if the image has already been cached (which is common). Here, the author uses the onload handler on an img element to catch the load event: < img src = "games.png" alt = "Games" onload = "gamesLogoHasLoaded(event)" > If the element is being added by script, then so long as the event handlers are added in the same script, the event will still not be missed: < script > var img = new Image (); img . src = 'games.png' ; img . alt = 'Games' ; img . onload = gamesLogoHasLoaded ; // img.addEventListener('load', gamesLogoHasLoaded, false); // would work also </ script > However, if the author first created the img element and then in a separate script added the event listeners, there's a chance that the load event would be fired in between, leading it to be missed: <!-- Do not use this style, it has a race condition! --> < img id = "games" src = "games.png" alt = "Games" > <!-- the 'load' event might fire here while the parser is taking a break, in which case you will not see it! --> < script > var img = document . getElementById ( 'games' ); img . onload = gamesLogoHasLoaded ; // might never fire! </ script >

1.10.3 How to catch mistakes when writing HTML: validators and conformance checkers

This section is non-normative.

Authors are encouraged to make use of conformance checkers (also known as validators) to catch common mistakes. The WHATWG maintains a list of such tools at: https://whatwg.org/validator/

1.11 Conformance requirements for authors

This section is non-normative.

Unlike previous versions of the HTML specification, this specification defines in some detail the required processing for invalid documents as well as valid documents.

However, even though the processing of invalid content is in most cases well-defined, conformance requirements for documents are still important: in practice, interoperability (the situation in which all implementations process particular content in a reliable and identical or equivalent way) is not the only goal of document conformance requirements. This section details some of the more common reasons for still distinguishing between a conforming document and one with errors.

1.11.1 Presentational markup

This section is non-normative.

The majority of presentational features from previous versions of HTML are no longer allowed. Presentational markup in general has been found to have a number of problems:

The use of presentational elements leads to poorer accessibility While it is possible to use presentational markup in a way that provides users of assistive technologies (ATs) with an acceptable experience (e.g. using ARIA), doing so is significantly more difficult than doing so when using semantically-appropriate markup. Furthermore, even using such techniques doesn't help make pages accessible for non-AT non-graphical users, such as users of text-mode browsers. Using media-independent markup, on the other hand, provides an easy way for documents to be authored in such a way that they work for more users (e.g. users of text browsers). Higher cost of maintenance It is significantly easier to maintain a site written in such a way that the markup is style-independent. For example, changing the color of a site that uses <font color=""> throughout requires changes across the entire site, whereas a similar change to a site based on CSS can be done by changing a single file. Larger document sizes Presentational markup tends to be much more redundant, and thus results in larger document sizes.

For those reasons, presentational markup has been removed from HTML in this version. This change should not come as a surprise; HTML4 deprecated presentational markup many years ago and provided a mode (HTML4 Transitional) to help authors move away from presentational markup; later, XHTML 1.1 went further and obsoleted those features altogether.

The only remaining presentational markup features in HTML are the style attribute and the style element. Use of the style attribute is somewhat discouraged in production environments, but it can be useful for rapid prototyping (where its rules can be directly moved into a separate style sheet later) and for providing specific styles in unusual cases where a separate style sheet would be inconvenient. Similarly, the style element can be useful in syndication or for page-specific styles, but in general an external style sheet is likely to be more convenient when the styles apply to multiple pages.

It is also worth noting that some elements that were previously presentational have been redefined in this specification to be media-independent: b , i , hr , s , small , and u .

1.11.2 Syntax errors

This section is non-normative.

The syntax of HTML is constrained to avoid a wide variety of problems.

Unintuitive error-handling behavior Certain invalid syntax constructs, when parsed, result in DOM trees that are highly unintuitive. For example, the following markup fragment results in a DOM with an hr element that is an earlier sibling of the corresponding table element: < table >< hr > ... Errors with optional error recovery To allow user agents to be used in controlled environments without having to implement the more bizarre and convoluted error handling rules, user agents are permitted to fail whenever encountering a parse error. Errors where the error-handling behavior is not compatible with streaming user agents Some error-handling behavior, such as the behavior for the <table><hr>... example mentioned above, are incompatible with streaming user agents (user agents that process HTML files in one pass, without storing state). To avoid interoperability problems with such user agents, any syntax resulting in such behavior is considered invalid. Errors that can result in infoset coercion When a user agent based on XML is connected to an HTML parser, it is possible that certain invariants that XML enforces, such as element or attribute names never contain multiple colons, will be violated by an HTML file. Handling this can require that the parser coerce the HTML DOM into an XML-compatible infoset. Most syntax constructs that require such handling are considered invalid. (Comments containing two consecutive hyphens, or ending with a hyphen, are exceptions that are allowed in the HTML syntax.) Errors that result in disproportionately poor performance Certain syntax constructs can result in disproportionately poor performance. To discourage the use of such constructs, they are typically made non-conforming. For example, the following markup results in poor performance, since all the unclosed i elements have to be reconstructed in each paragraph, resulting in progressively more elements in each paragraph: < p >< i > She dreamt. < p >< i > She dreamt that she ate breakfast. < p >< i > Then lunch. < p >< i > And finally dinner. The resulting DOM for this fragment would be: p i #text : She dreamt.

p i i #text : She dreamt that she ate breakfast.

p i i i #text : Then lunch.

p i i i i #text : And finally dinner.

Errors involving fragile syntax constructs There are syntax constructs that, for historical reasons, are relatively fragile. To help reduce the number of users who accidentally run into such problems, they are made non-conforming. For example, the parsing of certain named character references in attributes happens even with the closing semicolon being omitted. It is safe to include an ampersand followed by letters that do not form a named character reference, but if the letters are changed to a string that does form a named character reference, they will be interpreted as that character instead. In this fragment, the attribute's value is " ?bill&ted ": < a href = "?bill&ted" > Bill and Ted </ a > In the following fragment, however, the attribute's value is actually " ?art© ", not the intended " ?art© ", because even without the final semicolon, " © " is handled the same as " © " and thus gets interpreted as " © ": < a href = "?art©" > Art and Copy </ a > To avoid this problem, all named character references are required to end with a semicolon, and uses of named character references without a semicolon are flagged as errors. Thus, the correct way to express the above cases is as follows: < a href = "?bill&ted" > Bill and Ted </ a > <!-- &ted is ok, since it's not a named character reference --> < a href = "?art&copy" > Art and Copy </ a > <!-- the & has to be escaped, since © is a named character reference --> Errors involving known interoperability problems in legacy user agents Certain syntax constructs are known to cause especially subtle or serious problems in legacy user agents, and are therefore marked as non-conforming to help authors avoid them. For example, this is why the U+0060 GRAVE ACCENT character (`) is not allowed in unquoted attributes. In certain legacy user agents, it is sometimes treated as a quote character. Another example of this is the DOCTYPE, which is required to trigger no-quirks mode, because the behavior of legacy user agents in quirks mode is often largely undocumented. Errors that risk exposing authors to security attacks Certain restrictions exist purely to avoid known security problems. For example, the restriction on using UTF-7 exists purely to avoid authors falling prey to a known cross-site-scripting attack using UTF-7. [UTF7] Cases where the author's intent is unclear Markup where the author's intent is very unclear is often made non-conforming. Correcting these errors early makes later maintenance easier. For example, it is unclear whether the author intended the following to be an h1 heading or an h2 heading: < h1 > Contact details </ h2 > Cases that are likely to be typos When a user makes a simple typo, it is helpful if the error can be caught early, as this can save the author a lot of debugging time. This specification therefore usually considers it an error to use element names, attribute names, and so forth, that do not match the names defined in this specification. For example, if the author typed <capton> instead of <caption> , this would be flagged as an error and the author could correct the typo immediately. Errors that could interfere with new syntax in the future In order to allow the language syntax to be extended in the future, certain otherwise harmless features are disallowed. For example, "attributes" in end tags are ignored currently, but they are invalid, in case a future change to the language makes use of that syntax feature without conflicting with already-deployed (and valid!) content.

Some authors find it helpful to be in the practice of always quoting all attributes and always including all optional tags, preferring the consistency derived from such custom over the minor benefits of terseness afforded by making use of the flexibility of the HTML syntax. To aid such authors, conformance checkers can provide modes of operation wherein such conventions are enforced.

1.11.3 Restrictions on content models and on attribute values

This section is non-normative.

Beyond the syntax of the language, this specification also places restrictions on how elements and attributes can be specified. These restrictions are present for similar reasons:

Errors involving content with dubious semantics To avoid misuse of elements with defined meanings, content models are defined that restrict how elements can be nested when such nestings would be of dubious value. For example, this specification disallows nesting a section element inside a kbd element, since it is highly unlikely for an author to indicate that an entire section should be keyed in. Errors that involve a conflict in expressed semantics Similarly, to draw the author's attention to mistakes in the use of elements, clear contradictions in the semantics expressed are also considered conformance errors. In the fragments below, for example, the semantics are nonsensical: a separator cannot simultaneously be a cell, nor can a radio button be a progress bar. < hr role = "cell" > < input type = radio role = progressbar > Another example is the restrictions on the content models of the ul element, which only allows li element children. Lists by definition consist just of zero or more list items, so if a ul element contains something other than an li element, it's not clear what was meant. Cases where the default styles are likely to lead to confusion Certain elements have default styles or behaviors that make certain combinations likely to lead to confusion. Where these have equivalent alternatives without this problem, the confusing combinations are disallowed. For example, div elements are rendered as block boxes, and span elements as inline boxes. Putting a block box in an inline box is unnecessarily confusing; since either nesting just div elements, or nesting just span elements, or nesting span elements inside div elements all serve the same purpose as nesting a div element in a span element, but only the latter involves a block box in an inline box, the latter combination is disallowed. Another example would be the way interactive content cannot be nested. For example, a button element cannot contain a textarea element. This is because the default behavior of such nesting interactive elements would be highly confusing to users. Instead of nesting these elements, they can be placed side by side. Errors that indicate a likely misunderstanding of the specification Sometimes, something is disallowed because allowing it would likely cause author confusion. For example, setting the disabled attribute to the value " false " is disallowed, because despite the appearance of meaning that the element is enabled, it in fact means that the element is disabled (what matters for implementations is the presence of the attribute, not its value). Errors involving limits that have been imposed merely to simplify the language Some conformance errors simplify the language that authors need to learn. For example, the area element's shape attribute, despite accepting both circ and circle values in practice as synonyms, disallows the use of the circ value, so as to simplify tutorials and other learning aids. There would be no benefit to allowing both, but it would cause extra confusion when teaching the language. Errors that involve peculiarities of the parser Certain elements are parsed in somewhat eccentric ways (typically for historical reasons), and their content model restrictions are intended to avoid exposing the author to these issues. For example, a form element isn't allowed inside phrasing content, because when parsed as HTML, a form element's start tag will imply a p element's end tag. Thus, the following markup results in two paragraphs, not one: < p > Welcome. < form >< label > Name: </ label > < input ></ form > It is parsed exactly like the following: < p > Welcome. </ p >< form >< label > Name: </ label > < input ></ form > Errors that would likely result in scripts failing in hard-to-debug ways Some errors are intended to help prevent script problems that would be hard to debug. This is why, for instance, it is non-conforming to have two id attributes with the same value. Duplicate IDs lead to the wrong element being selected, with sometimes disastrous effects whose cause is hard to determine. Errors that waste authoring time Some constructs are disallowed because historically they have been the cause of a lot of wasted authoring time, and by encouraging authors to avoid making them, authors can save time in future efforts. For example, a script element's src attribute causes the element's contents to be ignored. However, this isn't obvious, especially if the element's contents appear to be executable script — which can lead to authors spending a lot of time trying to debug the inline script without realizing that it is not executing. To reduce this problem, this specification makes it non-conforming to have executable script in a script element when the src attribute is present. This means that authors who are validating their documents are less likely to waste time with this kind of mistake. Errors that involve areas that affect authors migrating between the HTML and XML syntaxes Some authors like to write files that can be interpreted as both XML and HTML with similar results. Though this practice is discouraged in general due to the myriad of subtle complications involved (especially when involving scripting, styling, or any kind of automated serialization), this specification has a few restrictions intended to at least somewhat mitigate the difficulties. This makes it easier for authors to use this as a transitionary step when migrating between the HTML and XML syntaxes. For example, there are somewhat complicated rules surrounding the lang and xml:lang attributes intended to keep the two synchronized. Another example would be the restrictions on the values of xmlns attributes in the HTML serialization, which are intended to ensure that elements in conforming documents end up in the same namespaces whether processed as HTML or XML. Errors that involve areas reserved for future expansion As with the restrictions on the syntax intended to allow for new syntax in future revisions of the language, some restrictions on the content models of elements and values of attributes are intended to allow for future expansion of the HTML vocabulary. For example, limiting the values of the target attribute that start with an U+005F LOW LINE character (_) to only specific predefined values allows new predefined values to be introduced at a future time without conflicting with author-defined values. Errors that indicate a mis-use of other specifications Certain restrictions are intended to support the restrictions made by other specifications. For example, requiring that attributes that take media query lists use only valid media query lists reinforces the importance of following the conformance rules of that specification.

1.12 Suggested reading

This section is non-normative.

The following documents might be of interest to readers of this specification.

Character Model for the World Wide Web 1.0: Fundamentals [CHARMOD] This Architectural Specification provides authors of specifications, software developers, and content developers with a common reference for interoperable text manipulation on the World Wide Web, building on the Universal Character Set, defined jointly by the Unicode Standard and ISO/IEC 10646. Topics addressed include use of the terms 'character', 'encoding' and 'string', a reference processing model, choice and identification of character encodings, character escaping, and string indexing. Unicode Security Considerations [UTR36] Because Unicode contains such a large number of characters and incorporates the varied writing systems of the world, incorrect usage can expose programs or systems to possible security attacks. This is especially important as more and more products are internationalized. This document describes some of the security considerations that programmers, system analysts, standards developers, and users should take into account, and provides specific recommendations to reduce the risk of problems. Web Content Accessibility Guidelines (WCAG) 2.0 [WCAG] Web Content Accessibility Guidelines (WCAG) 2.0 covers a wide range of recommendations for making web content more accessible. Following these guidelines will make content accessible to a wider range of people with disabilities, including blindness and low vision, deafness and hearing loss, learning disabilities, cognitive limitations, limited movement, speech disabilities, photosensitivity and combinations of these. Following these guidelines will also often make your web content more usable to users in general. Authoring Tool Accessibility Guidelines (ATAG) 2.0 [ATAG] This specification provides guidelines for designing web content authoring tools that are more accessible for people with disabilities. An authoring tool that conforms to these guidelines will promote accessibility by providing an accessible user interface to authors with disabilities as well as by enabling, supporting, and promoting the production of accessible web content by all authors. User Agent Accessibility Guidelines (UAAG) 2.0 [UAAG] This document provides guidelines for designing user agents that lower barriers to web accessibility for people with disabilities. User agents include browsers and other types of software that retrieve and render web content. A user agent that conforms to these guidelines will promote accessibility through its own user interface and through other internal facilities, including its ability to communicate with other technologies (especially assistive technologies). Furthermore, all users, not just users with disabilities, should find conforming user agents to be more usable.

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 Document 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 Document 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 text/html , and the term XML document refers to resources labeled with an XML MIME type.

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:

Let parallelQueue be a new parallel queue. Run the following steps in parallel: While true: Let steps be the result of dequeueing from parallelQueue 's algorithm queue. If steps is not nothing, then run steps . Assert: running steps did not throw an exception, as steps running in parallel are not allowed to throw. Implementations are not expected to implement this as a continuously running loop. Algorithms in standards are to be easy to understand and are not necessarily great for battery life or performance. Return parallelQueue .

Steps running in parallel can themselves run other steps in in parallel. E.g., inside a parallel queue it can be useful to run a series of steps in parallel with the queue.

Imagine a standard defined nameList (a list), along with a method to add a name to nameList , unless nameList already contains name , in which case it rejects. The following solution suffers from race conditions: Let p be a new promise. Run the following steps in parallel: If nameList contains name , reject p with a TypeError and abort these steps. Do some potentially lengthy work. Append name to nameList . Resolve p with undefined. Return p . Two invocations of the above could run simultaneously, meaning name isn't in nameList during step 2.1, but it might be added before step 2.3 runs, meaning name ends up in nameList twice. Parallel queues solve this. The standard would let nameListQueue be the result of starting a new parallel queue, then: Let p be a new promise. Enqueue the following steps to nameListQueue : If nameList contains name , reject p with a TypeError and abort these steps. Do some potentially lengthy work. Append name to nameList . Resolve p with undefined. 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, UAs conforming to this specification will place elements in HTML in the http://www.w3.org/1999/xhtml 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, button elements are elements with the element type button , 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 Name 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, Text node, or string, means that the length of the text is zero (i.e., not even containing controls or U+0020 SPACE).

A node A is inserted into a node B when the insertion steps are invoked with A as the argument and A 's new parent is B . Similarly, a node A is removed from a node B when the removing steps are invoked with A as the removedNode argument and B as the oldParent argument.

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 Document object, but that neither act as child browsing contexts of the Document nor introduce any Node objects to the Document '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 text/plain and application/octet-stream as having a registered plugin.

One example of a plugin would be a PDF viewer that is instantiated in a browsing context 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]

A plugin can be secured if it honors the semantics of the sandbox attribute.

For example, a secured plugin would prevent its contents from creating popups when the plugin is instantiated inside a sandboxed iframe .

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.

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 agents Web browsers that support the XML syntax must process elements and attributes from the HTML namespace found in XML documents as described in this specification, so that users can interact with them, unless the semantics of those elements have been overridden by other specifications. A conforming web browser would, upon finding a 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 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 script element in the example above would still implement the HTMLScriptElement interface. Non-interactive presentation user agents User agents that process HTML and XML documents purely to render non-interactive versions of them must comply to the same conformance criteria as web browsers, except that they are exempt from requirements regarding user interaction. Typical examples of non-interactive presentation user agents are printers (static UAs) and overhead displays (dynamic UAs). It is expected that most static non-interactive presentation user agents will also opt to lack scripting support. A non-interactive but dynamic presentation UA would still execute scripts, allowing forms to be dynamically submitted, and so forth. However, since the concept of "focus" is irrelevant when the user cannot interact with the document, the UA would not need to support any of the focus-related DOM APIs. 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 the rendering 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: Criteria that can be expressed in a DTD. Criteria that cannot be expressed by a DTD, but can still be checked by a machine. 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 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 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 its nearest article or body element ancestor. However, since an authoring tool is likely unable to determine the difference, an authoring tool is exempt from that requirement. This does not mean, though, that authoring tools can use address elements for any block of italics text (for instance); it just means that the authoring tool doesn't have to verify that when the user uses a tool for inserting contact information for an article element, that the user really is doing that and not inserting something else instead. 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, 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

✔ MDN MessageEvent/MessageEvent Support in all current engines. Firefox 4+ Safari 4+ Chrome 1+ Opera ? Edge 79+ Edge (Legacy) 12+ Internet Explorer 9+ Firefox Android ? Safari iOS 3+ Chrome Android 18+ WebView Android 37+ Samsung Internet 1.0+ Opera Android ?

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 elements array would be in violation of the above requirement, as it would violate the definition of elements given in this specification.

When vendor-neutral extensions to this specification are needed, either this specification can be updated accordingly, or an extension specification can be written that overrides the requirements in this specification. When someone applying this specification to their activities decides that they will recognize the requirements of such an extension specification, it becomes an applicable specification for the purposes of conformance requirements in this specification.

Someone could write a specification that defines any arbitrary byte stream as conforming, and then claim that their random junk is conforming. However, that does not mean that their random junk actually is conforming for everyone's purposes: if someone else decides that that specification does not apply to their work, then they can quite legitimately say that the aforementioned random junk is just that, junk, and not conforming at all. As far as conformance goes, what matters in a particular community is what that community agrees is applicable.

User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them.

When support for a feature is disabled (e.g. as an emergency measure to mitigate a security problem, or to aid in development, or for performance reasons), user agents must act as if they had no support for the feature whatsoever, and as if the feature was not mentioned in this specification. For example, if a particular feature is accessed via an attribute in a Web IDL interface, the attribute itself would be omitted from the objects that implement that interface — leaving the attribute on the object but making it return null or throw an exception is insufficient.

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: If the context node is from an HTML DOM, the default element namespace is "http://www.w3.org/1999/xhtml". 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]

XSLT 1.0 processors outputting to a DOM when the output method is "html" (either explicitly or via the defaulting rule in XSLT 1.0) are affected as follows:

If the transformation program outputs an element in no namespace, the processor must, prior to constructing the corresponding DOM element node, change the namespace of the element to the HTML namespace, ASCII-lowercase the element's local name, and ASCII-lowercase the names of any non-namespaced attributes on the element.

This requirement is a willful violation of the XSLT 1.0 specification, required because this specification changes the namespaces and case-sensitivity rules of HTML in a manner that would otherwise be incompatible with DOM-based XSLT transformations. (Processors that serialize the output are unaffected.) [XSLT10]

This specification does not specify precisely how XSLT processing interacts with the HTML parser infrastructure (for example, whether an XSLT processor acts as if it puts any elements into a stack of open elements). However, XSLT processors must stop parsing if they successfully complete, and must set the current document readiness first to " interactive " and then to " complete " if they are aborted.

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.

2.2 String comparison

A string pattern is a prefix match for a string s when pattern is not longer than s and truncating s to pattern 's length leaves the two strings as matches of each other.

2.3 Policy-controlled features

This document defines the following policy-controlled features:

MDN Headers/Feature-Policy/document-domain Firefox 74+ Safari No Chrome 77+ Opera 64+ Edge 79+ Edge (Legacy) No Internet Explorer No Firefox Android 🔰 65+ Safari iOS No Chrome Android No WebView Android No Samsung Internet No Opera Android No

" autoplay ", which has a default allowlist of 'self' .

", which has a default allowlist of . " cross-origin-isolated ", which has a default allowlist of 'self' .

", which has a default allowlist of . " document-domain ", which has a default allowlist of * .

2.4 Common microsyntaxes

There are various places in HTML that accept particular data types, such as dates or numbers. This section describes what the conformance criteria for content in those formats is, and how to parse them.

Implementors are strongly urged to carefully examine any third-party libraries they might consider using to implement the parsing of syntaxes described below. For example, date libraries are likely to implement error handling behavior that differs from what is required in this specification, since error-handling behavior is often not defined in specifications that describe date syntaxes similar to those used in this specification, and thus implementations tend to vary greatly in how they handle errors.

2.4.1 Common parser idioms

Some of the micro-parsers described below follow the pattern of having an input variable that holds the string being parsed, and having a position variable pointing at the next character to parse in input .

2.4.2 Boolean attributes

A number of attributes are boolean attributes . The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.

Here is an example of a checkbox that is checked and disabled. The checked and disabled attributes are the boolean attributes. < label >< input type = checkbox checked name = cheese disabled > Cheese </ label > This could be equivalently written as this: < label >< input type = checkbox checked = checked name = cheese disabled = disabled > Cheese </ label > You can also mix styles; the following is still equivalent: < label >< input type = 'checkbox' checked name = cheese disabled = "" > Cheese </ label >

2.4.3 Keywords and enumerated attributes

Some attributes are defined as taking one of a finite set of keywords. Such attributes are called enumerated attributes . The keywords are each defined to map to a particular state (several keywords might map to the same state, in which case some of the keywords are synonyms of each other; additionally, some of the keywords can be said to be non-conforming, and are only in the specification for historical reasons). In addition, two default states can be given. The first is the invalid value default , the second is the missing value default .

If an enumerated attribute is specified, the attribute's value must be an ASCII case-insensitive match for one of the given keywords that are not said to be non-conforming, with no leading or trailing whitespace.

When the attribute is specified, if its value is an ASCII case-insensitive match for one of the given keywords then that keyword's state is the state that the attribute represents. If the attribute value matches none of the given keywords, but the attribute has an invalid value default, then the attribute represents that state. Otherwise, there is no default, and invalid values mean that there is no state represented.

When the attribute is not specified, if there is a missing value default state defined, then that is the state represented by the (missing) attribute. Otherwise, the absence of the attribute means that there is no state represented.

The empty string can be a valid keyword.

2.4.4 Numbers

2.4.4.1 Signed integers

A string is a valid integer if it consists of one or more ASCII digits, optionally prefixed with a U+002D HYPHEN-MINUS character (-).

A valid integer without a U+002D HYPHEN-MINUS (-) prefix represents the number that is represented in base ten by that string of digits. A valid integer with a U+002D HYPHEN-MINUS (-) prefix represents the number represented in base ten by the string of digits that follows the U+002D HYPHEN-MINUS, subtracted from zero.

The rules for parsing integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either an integer or an error.

Let input be the string being parsed. Let position be a pointer into input , initially pointing at the start of the string. Let sign have the value "positive". Skip ASCII whitespace within input given position . If position is past the end of input , return an error. If the character indicated by position (the first character) is a U+002D HYPHEN-MINUS character (-): Let sign be "negative". Advance position to the next character. If position is past the end of input , return an error. Otherwise, if the character indicated by position (the first character) is a U+002B PLUS SIGN character (+): Advance position to the next character. (The " + " is ignored, but it is not conforming.) If position is past the end of input , return an error. If the character indicated by position is not an ASCII digit, then return an error. Collect a sequence of code points that are ASCII digits from input given position , and interpret the resulting sequence as a base-ten integer. Let value be that integer. If sign is "positive", return value , otherwise return the result of subtracting value from zero.

2.4.4.2 Non-negative integers

A string is a valid non-negative integer if it consists of one or more ASCII digits.

A valid non-negative integer represents the number that is represented in base ten by that string of digits.

The rules for parsing non-negative integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either zero, a positive integer, or an error.

Let input be the string being parsed. Let value be the result of parsing input using the rules for parsing integers. If value is an error, return an error. If value is less than zero, return an error. Return value .

2.4.4.3 Floating-point numbers

A string is a valid floating-point number if it consists of:

Optionally, a U+002D HYPHEN-MINUS character (-). One or both of the following, in the given order: A series of one or more ASCII digits. Both of the following, in the given order: A single U+002E FULL STOP character (.). A series of one or more ASCII digits. Optionally: Either a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E). Optionally, a U+002D HYPHEN-MINUS character (-) or U+002B PLUS SIGN character (+). A series of one or more ASCII digits.

A valid floating-point number represents the number obtained by multiplying the significand by ten raised to the power of the exponent, where the significand is the first number, interpreted as base ten (including the decimal point and the number after the decimal point, if any, and interpreting the significand as a negative number if the whole string starts with a U+002D HYPHEN-MINUS character (-) and the number is not zero), and where the exponent is the number after the E, if any (interpreted as a negative number if there is a U+002D HYPHEN-MINUS character (-) between the E and the number and the number is not zero, or else ignoring a U+002B PLUS SIGN character (+) between the E and the number if there is one). If there is no E, then the exponent is treated as zero.

The Infinity and Not-a-Number (NaN) values are not valid floating-point numbers.

The valid floating-point number concept is typically only used to restrict what is allowed for authors, while the user agent requirements use the rules for parsing floating-point number values below (e.g., the max attribute of the progress element). However, in some cases the user agent requirements include checking if a string is a valid floating-point number (e.g., the value sanitization algorithm for the Number state of the input element, or the parse a srcset attribute algorithm).

The best representation of the number n as a floating-point number is the string obtained from running ToString( n ). The abstract operation ToString is not uniquely determined. When there are multiple possible strings that could be obtained from ToString for a particular value, the user agent must always return the same string for that value (though it may differ from the value used by other user agents).

The rules for parsing floating-point number values are as given in the following algorithm. This algorithm must be aborted at the first step that returns something. This algorithm will return either a number or an error.

Let input be the string being parsed. Let position be a pointer into input , initially pointing at the start of the string. Let value have the value 1. Let divisor have the value 1. Let exponent have the value 1. Skip ASCII whitespace within input given position . If position is past the end of input , return an error. If the character indicated by position is a U+002D HYPHEN-MINUS character (-): Change value and divisor to −1. Advance position to the next character. If position is past the end of input , return an error. Otherwise, if the character indicated by position (the first character) is a U+002B PLUS SIGN character (+): Advance position to the next character. (The " + " is ignored, but it is not conforming.) If position is past the end of input , return an error. If the character indicated by position is a U+002E FULL STOP (.), and that is not the last character in input , and the character after the character indicated by position is an ASCII digit, then set value to zero and jump to the step labeled fraction. If the character indicated by position is not an ASCII digit, then return an error. Collect a sequence of code points that are ASCII digits from input given position , and interpret the resulting sequence as a base-ten integer. Multiply value by that integer. If position is past the end of input , jump to the step labeled conversion. Fraction: If the character indicated by position is a U+002E FULL STOP (.), run these substeps: Advance position to the next character. If position is past the end of input , or if the character indicated by position is not an ASCII digit, U+0065 LATIN SMALL LETTER E (e), or U+0045 LATIN CAPITAL LETTER E (E), then jump to the step labeled conversion. If the character indicated by position is a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E), skip the remainder of these substeps. Fraction loop: Multiply divisor by ten. Add the value of the character indicated by position , interpreted as a base-ten digit (0..9) and divided by divisor , to value . Advance position to the next character. If position is past the end of input , then jump to the step labeled conversion. If the character indicated by position is an ASCII digit, jump back to the step labeled fraction loop in these substeps. If the character indicated by position is U+0065 (e) or a U+0045 (E), then: Advance position to the next character. If position is past the end of input , then jump to the step labeled conversion. If the character indicated by position is a U+002D HYPHEN-MINUS character (-): Change exponent to −1. Advance position to the next character. If position is past the end of input , then jump to the step labeled conversion. Otherwise, if the character indicated by position is a U+002B PLUS SIGN character (+): Advance position to the next character. If position is past the end of input , then jump to the step labeled conversion. If the character indicated by position is not an ASCII digit, then jump to the step labeled conversion. Collect a sequence of code points that are ASCII digits from input given position , and interpret the resulting sequence as a base-ten integer. Multiply exponent by that integer. Multiply value by ten raised to the exponent th power. Conversion: Let S be the set of finite IEEE 754 double-precision floating-point values except −0, but with two special values added: 21024 and −21024. Let rounded-value be the number in S that is closest to value , selecting the number with an even significand if there are two equally close values. (The two special values 21024 and −21024 are considered to have even significands for this purpose.) If rounded-value is 21024 or −21024, return an error. Return rounded-value .

2.4.4.4 Percentages and lengths

The rules for parsing dimension values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a number greater than or equal to 0.0, or failure; if a number is returned, then it is further categorized as either a percentage or a length.

Let input be the string being parsed. Let position be a position variable for input , initially pointing at the start of input . Skip ASCII whitespace within input given position . If position is past the end of input or the code point at position within input is not an ASCII digit, then return failure. Collect a sequence of code points that are ASCII digits from input given position , and interpret the resulting sequence as a base-ten integer. Let value be that number. If position is past the end of input , then return value as a length. If the code point at position within input is U+002E (.), then: Advance position by 1. If position is past the end of input or the code point at position within input is not an ASCII digit, then return the current dimension value with value , input , and position . Let divisor have the value 1. While true: Multiply divisor by ten. Add the value of the code point at position within input , interpreted as a base-ten digit (0..9) and divided by divisor , to value . Advance position by 1. If position is past the end of input , then return value as a length. If the code point at position within input is not an ASCII digit, then break. Return the current dimension value with value , input , and position .

The current dimension value , given value , input , and position , is determined as follows:

If position is past the end of input , then return value as a length. If the code point at position within input is U+0025 (%), then return value as a percentage. Return value as a length.

2.4.4.5 Non-zero percentages and lengths

The rules for parsing nonzero dimension values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a number greater than 0.0, or an error; if a number is returned, then it is further categorized as either a percentage or a length.

Let input be the string being parsed. Let value be the result of parsing input using the rules for parsing dimension values. If value is an error, return an error. If value is zero, return an error. If value is a percentage, return value as a percentage. Return value as a length.

2.4.4.6 Lists of floating-point numbers

A valid list of floating-point numbers is a number of valid floating-point numbers separated by U+002C COMMA characters, with no other characters (e.g. no ASCII 