HTML5 Differences from HTML4

W3C Working Group Note 9 December 2014

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"HTML5 Differences from HTML4" describes the differences of the HTML5 specification from those of HTML4.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is the 9 December 2014 W3C Working Group Note produced by the HTML Working Group, part of the HTML Activity. The Working Group intends to publish this document as a Working Group Note. The appropriate forum for comments is W3C Bugzilla. (public-html-comments@w3.org, a mailing list with a public archive, is no longer used for tracking comments.)

Publication as a Working Group Note does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 14 October 2005 W3C Process Document.

Table of Contents

1 Introduction

1.1 Scope of This Document

This document covers the W3C HTML5 specification. It does not cover the W3C HTML5.1 specification or the WHATWG HTML standard. [HTML5] [HTML5NIGHTLY] [HTML]

1.2 History of HTML

HTML has been in continuous evolution since it was introduced to the Internet in the early 1990s. Some features were introduced in specifications; others were introduced in software releases. In some respects, implementations and Web developer practices have converged with each other and with specifications and standards, but in other ways, they have diverged.

HTML4 became a W3C Recommendation in 1997. While it continues to serve as a rough guide to many of the core features of HTML, it does not provide enough information to build implementations that interoperate with each other and, more importantly, with Web content. The same goes for XHTML1, which defines an XML serialization for HTML4, and DOM Level 2 HTML, which defines JavaScript APIs for both HTML and XHTML. HTML replaces these documents. [DOM2HTML] [HTML4] [XHTML1]

The HTML specification reflects an effort, started in 2004, to study contemporary HTML implementations and Web content. The specification:

  1. Defines a single language called HTML which can be written in HTML syntax and in XML syntax.

  2. Defines detailed processing models to foster interoperable implementations.

  3. Improves markup for documents.

  4. Introduces markup and APIs for emerging idioms, such as Web applications.

1.3 Open Issues

See the "Status of This Document" section of the HTML5 specification.

1.4 Backward Compatibility

HTML is defined in a way that is backward compatible with the way user agents handle content. To keep the language relatively simple for Web developers, several older elements and attributes are not included, as outlined in the other sections of this document, such as presentational elements that are better handled using CSS.

User agents, however, will always have to support these older elements and attributes. This is why the HTML specification clearly separates requirements for Web developers (referred to as "authors" in the specification) and user agents; for instance, this means that Web developers cannot use the isindex or the plaintext element, but user agents are required to support them in a way that is compatible with how these elements need to behave for compatibility with Web content.

Since HTML has separate conformance requirements for Web developers and user agents there is no longer a need for marking features "deprecated".

2 Syntax

HTML defines a syntax, referred to as "the HTML syntax", that is mostly compatible with HTML4 and XHTML1 documents published on the Web, but is not compatible with the more esoteric SGML features of HTML4, such as processing instructions and shorthand markup as these are not supported by most user agents. Documents using the HTML syntax are served with the text/html media type.

HTML also defines detailed parsing rules (including "error handling") for this syntax which are largely compatible with HTML4-era implementations. User agents have to use these rules for resources that have the text/html media type. Here is an example document that conforms to the HTML syntax:

<!doctype html>
    <meta charset="UTF-8">
    <title>Example document</title>
    <p>Example paragraph</p>

The other syntax that can be used for HTML is XML. This syntax is compatible with XHTML1 documents and implementations. Documents using this syntax need to be served with an XML media type (such as application/xhtml+xml or application/xml) and elements need to be put in the http://www.w3.org/1999/xhtml namespace following the rules set forth by the XML specifications. [XML] [XMLNS]

Below is an example document that conforms to the XML syntax of HTML.

<?xml version="1.0" encoding="UTF-8"?>
<html xmlns="http://www.w3.org/1999/xhtml">
    <title>Example document</title>
    <p>Example paragraph</p>

2.1 Character Encoding

For the HTML syntax, Web developers are required to declare the character encoding. There are three ways to do that:

For the XML syntax, Web developers have to use the rules as set forth in the XML specification to set the character encoding.

2.2 The Doctype

The HTML syntax requires a doctype to be specified to ensure that the browser renders the page in standards mode. The doctype has no other purpose. [DOCTYPE]

The doctype declaration for the HTML syntax is <!DOCTYPE html> and is case-insensitive. Doctypes from earlier versions of HTML were longer because the HTML language was SGML-based and therefore required a reference to a DTD. This is no longer the case and the doctype is only needed to enable standards mode for documents written using the HTML syntax. Browsers already do this for <!DOCTYPE html>.

To support legacy markup generators that cannot generate the preferred short doctype, the doctype <!DOCTYPE html SYSTEM "about:legacy-compat"> is allowed in the HTML syntax.

The strict doctypes for HTML 4.0, HTML 4.01, XHTML 1.0 as well as XHTML 1.1 are also allowed (but are discouraged) in the HTML syntax.

In the XML syntax, any doctype declaration may be used, or it may be omitted altogether. Documents with an XML media type are always handled in standards mode.

2.3 MathML and SVG

The HTML syntax allows for MathML and SVG elements to be used inside a document. An math or svg start tag causes the HTML parser to switch to a special insertion mode which puts elements and attributes in the appropriate namespaces, does case fixups for elements and attributes that have mixed case, and supports the empty-element syntax as in XML. The syntax is still case-insensitive and attributes allow the same syntax as for HTML elements. Namespace declarations may be omitted. CDATA sections are supported in this insertion mode.

Some MathML and SVG elements cause the parser to switch back to "HTML mode", e.g. mtext and foreignObject, so you can use HTML elements or a new math or svg element.

For instance, a very simple document using some of the minimal syntax features could look like:

<!doctype html>
<title>SVG in text/html</title>
 A green circle:
 <svg> <circle r="50" cx="50" cy="50" fill="green"/> </svg>

2.4 Miscellaneous

There are a few other changes in the HTML syntax worthy of mentioning:

3 Language

This section is split up in several subsections to more clearly illustrate the various differences from HTML4.

3.1 New Elements

The following elements have been introduced for better structure:

Then there are several other new elements:

The input element's type attribute now has the following new values:

The idea of these new types is that the user agent can provide the user interface, such as a calendar date picker or integration with the user's address book, and submit a defined format to the server. It gives the user a better experience as his input is checked before sending it to the server meaning there is less time to wait for feedback.

3.2 New Attributes

Several attributes have been introduced to various elements that were already part of HTML4:

Several attributes from HTML4 now apply to all elements. These are called global attributes: accesskey, class, dir, id, lang, style, tabindex and title. Additionally, XHTML 1.0 only allowed xml:space on some elements, which is now allowed on all elements in XHTML documents.

There are also several new global attributes:

HTML also makes all event handler attributes from HTML4, which take the form onevent, global attributes and adds several new event handler attributes for new events it defines; for instance, the onplay event handler attribute for the play event which is used by the API for the media elements (video and audio). The specification has an index of all events.

3.3 Changed Elements

These elements have slightly modified meanings in HTML to better reflect how they are used on the Web or to make them more useful:

3.4 Changed Attributes

Several attributes have changed in various ways.

3.5 Obsolete Elements

The elements in this section are not to be used by Web developers. User agents will still have to support them and various sections in HTML define how. E.g. the obsolete isindex element is handled by the parser section.

The following elements are not in HTML because their effect is purely presentational and their function is better handled by CSS:

The following elements are not in HTML because using them damages usability and accessibility:

The following elements are not included because they have not been used often, created confusion, or their function can be handled by other elements:

Finally the noscript element is only conforming in the HTML syntax. It is not allowed in the XML syntax. This is because in order to not only hide visually but also prevent the content to run scripts, apply style sheets, have submittable form controls, load resources, and so forth, the HTML parser parses the content of the noscript element as plain text. The same is not possible with an XML parser.

3.6 Obsolete Attributes

Some attributes from HTML4 are no longer allowed in HTML. The specification defines how user agents should process them in legacy documents, but Web developers are not allowed use them and they will not validate.

HTML has advice on what you can use instead.

In addition, HTML has none of the presentational attributes that were in HTML4 as their functions are better handled by CSS:

The following attributes are allowed but Web developers are discouraged from using them and instead strongly encouraged to use an alternative solution:

4 Content Model

The content model is what defines how elements may be nested — what is allowed as children (or descendants) of a certain element.

At a high level, HTML4 had two major categories of elements, "inline" (e.g. span, img, text), and "block-level" (e.g. div, hr, table). Some elements did not fit in either category.

Some elements allowed "inline" elements (e.g. p), some allowed "block-level" elements (e.g. body), some allowed both (e.g. div), while other elements did not allow either category but only allowed other specific elements (e.g. dl, table), or did not allow any children at all (e.g. link, img, hr).

Notice the difference between an element itself being in a certain category, and having a content model of a certain category; for instance, the p element is itself a "block-level" element, but has a content model of "inline".

To make it more confusing, HTML4 had different content model rules in its Strict, Transitional and Frameset flavors; for instance, in Strict, the body element allowed only "block-level" elements, but in Transitional, it allowed both "inline" and "block-level".

To make things more confusing still, CSS uses the terms "block-level element" and "inline-level element" for its visual formatting model, which is related to CSS's 'display' property and has nothing to do with HTML's content model rules.

HTML does not use the terms "block-level" or "inline" as part of its content model rules, to reduce confusion with CSS. However, it has more categories than HTML4, and an element can be part of none of them, one of them, or several of them.

As a broad change from HTML4, HTML no longer has any element that only accepts what HTML4 called "block-level" elements; e.g. the body element now allows flow content. Thus, This is closer to HTML4 Transitional than HTML4 Strict.

Further changes include:

5 APIs

HTML has introduced many new APIs and has extended, changed or obsoleted some existing APIs.

5.1 New APIs

HTML introduces a number of APIs that help in creating Web applications. These can be used together with the new elements introduced for applications:

5.2 Changed APIs

The following features from DOM Level 2 HTML are changed in various ways:

5.3 Extensions to Document

DOM Level 2 HTML had an HTMLDocument interface that inherited from Document and provided HTML-specific members on documents. HTML has moved these members to the Document interface, and extended it in a number of ways. Since all documents use the Document interface, the HTML-specific members are now available on all documents, so they are usable in e.g. SVG documents as well. It also has several new members:

Existing scripts that modified the prototype of HTMLDocument should continue to work because window.HTMLDocument now returns the Document interface object.

5.4 Extensions to HTMLElement

The HTMLElement interface has also gained several extensions in HTML:

Some members were previously defined on HTMLElement but been moved to the Element interface in the DOM standard: [DOM]

5.5 Extensions to Other Interfaces

Some interfaces in DOM Level 2 HTML have been extended.

In addition, most new content attributes also have corresponding IDL attributes on the elements' interfaces, e.g. the sizes IDL attribute on HTMLLinkElement which reflects the sizes content attribute.

5.6 Obsolete APIs

Some APIs are now either removed altogether, or marked as obsolete.

All IDL attributes that reflect a content attribute that is itself obsolete, are now also obsolete; for instance, the bgColor IDL attribute on HTMLBodyElement which reflects the obsolete bgcolor content attribute is now obsolete.

The following interfaces are marked obsolete since the elements are obsolete: HTMLAppletElement, HTMLFrameSetElement, HTMLFrameElement, HTMLDirectoryElement and HTMLFontElement.

The HTMLIsIndexElement interface is removed altogether since the HTML parser expands an isindex tag into other elements. The HTMLBaseFontElement interface is also removed since the element has no effect.

The following members of the HTMLDocument interface (which have now moved to Document) are now obsolete: anchors and applets.


The editors would like to thank Ben Millard, Bruce Lawson, Cameron McCormack, Charles McCathieNevile, Dan Connolly, David Håsäther, Dennis German, Frank Ellermann, Frank Palinkas, 羽田野太巳 (Futomi Hatano), Gordon P. Hemsley, Henri Sivonen, James Graham, Jens O. Meiert, Jeremy Keith, Jukka K. Korpela, Jürgen Jeka, Krijn Hoetmer, Leif Halvard Silli, Maciej Stachowiak, Mallory van Achterberg, Marcos Caceres, Mark Pilgrim, Martijn Wargers, Martin Leese, Martyn Haigh, Masataka Yakura, Michael Smith, Mike Taylor, Ms2ger, Olivier Gendrin, Øistein E. Andersen, Philip Jägenstedt, Philip Taylor, Randy Peterman, Robin Berjon, Steve Faulkner, Toby Inkster, Xaxio Brandish, Yngve Spjeld Landro and Zhong Yu for their contributions to this document as well as to all the people who have contributed to HTML over the years for improving the Web!


CSS Object Model (CSSOM), Simon Pieters and Glenn Adams. W3C.
Activating Browser Modes with Doctype, Henri Sivonen.
DOM, Anne van Kesteren, Aryeh Gregor and Ms2ger. WHATWG.
Document Object Model (DOM) Level 2 HTML Specification, Johnny Stenback, Philippe Le Hégaret and Arnaud Le Hors. W3C.
HTML, Ian Hickson. WHATWG.
HTML 4.01 Specification, Dave Raggett, Arnaud Le Hors and Ian Jacobs. W3C.
HTML5, Ian Hickson, Robin Berjon, Steve Faulkner, Travis Leithead et al.. W3C.
HTML 5.1 Nightly, Robin Berjon, Steve Faulkner, Travis Leithead et al.. W3C.
XHTML™ 1.1 - Module-based XHTML - Second Edition, Murray Altheim and Shane McCarron.
Extensible Markup Language, Tim Bray, Jean Paoli, C. M. Sperberg-McQueen et al.. W3C.
Namespaces in XML, Tim Bray, Dave Hollander, Andrew Layman et al.. W3C.