What is XHTML?

The Extensible HyperText Markup Language (XHTML™) is a family of currentand future document types and modules that reproduce, subset, and extend HTML,reformulated in XML rather than SGML. XHTML Familydocument types are all XML-based, and ultimately are designed to work inconjunction with XML-based user agents. XHTML is the successor of HTML, and aseries of specifications has been developed forXHTML. See also: HTML and XHTML Frequently AnsweredQuestions

Recommendations

W3C produces what are known as 'Recommendations'. These arespecifications, developed by W3C working groups, and then reviewed by Membersof the Consortium. A W3C Recommendation indicates that consensus has beenreached among the Consortium Members that a specification is appropriate forwidespread use.

In general, XHTML specifications include implementations of theirrequirements in various syntaxes (e.g., XML DTD, XML Schema, RelaxNG). Theseimplementations are normative, and are meant to be used either as buildingblocks for new markup languages (e.g., XHTML Modularization) or as completemarkup language implementations (e.g., XHTML 1.1).

While a normative part of the W3C Recommendation in which they arepresented, these implementations are also code containing potential errors oromissions. When such errors are discovered, it is sometimes important that theybe addressed very quickly to ensure that technologies relying on theimplementations work as expected (e.g., validators and content authoringsystems). The W3C process allows for the publication and frequent updating oferrata, but unfortunately this process does not enable implementations to bequickly updated. As a result, the XHTML 2 Working Group has adopted thefollowing concerning the production and evolution of its implementations:

• All implementations will adhere to the naming convention(s) and evolution rules as defined in XHTML Modularization. These names include both Formal Public Identifiers and System Identifiers. These conventions require that the System Identifier must include a revision number. This revision number is ONLY incremented when a revision is not backward compatible.
• Each applicable Recommendation will include fixed, unchanging versions of those implementations within the formal dated location for the Recommendation (/TR/YYYY/REC-whatever-YYYYmmdd/...).
• The Working Group will also provide a version of that implementation in the working group's space on the W3C server (/MarkUp), uncoupled from a specific dated version of the associated Recommendation. In the beginning this uncoupled version will be *identical* to the version from the associated Recommendation.
• If the Working Group identifies a problem with an implementation, and it is possible to solve the problem in a way that is 100 percent backward compatible, then the version in the group's space will be updated in place and an announcement will be sent to the XHTML 2 public email list.

The XHTML 2 Working Group states that the term 'backward compatible' shouldbe used only when:

• The external interface to the module cannot change in any way that would break another module or markup language, either within or outside of the W3C.
• The content model cannot change in any way that would cause a previously valid document to become invalid.

If either of the above constraints would be violated by a change, theworking group will either 1) not make the change, or 2) revise the applicablemodule. In the latter case, the working group will also change the associatedidentifiers.

XHTML 1.0 HTML4.01 XHTML basic Modularization of XHTML XHTML 1.1 XML Events

XHTML 1.0

XHTML 1.0 was the W3C's first Recommendation for XHTML, following on from earlier work on HTML 4.01, HTML 4.0, HTML 3.2 and HTML2.0. With a wealth of features, XHTML 1.0 is a reformulation of HTML 4.01 inXML, and combines the strength of HTML 4 with the power of XML.

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XHTML 1.0 was the first major change to HTML since HTML 4.0 was released in1997. It brings the rigor of XML to Web pages and is the keystone in W3C's workto create standards that provide richer Web pages on an ever increasing rangeof browser platforms including cell phones, televisions, cars, wallet sizedwireless communicators, kiosks, and desktops.

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XHTML 1.0 was the first step: it reformulates HTML as an XML application.This makes it easier to process and easier to maintain. XHTML 1.0 borrowselements and attributes from W3C's earlier work on HTML 4, and can beinterpreted by existing browsers, by following a few simple guidelines. This allows you to start usingXHTML now!

You can roll over your old HTML documents into XHTML using an Open Source HTML Tidy utility. This tool also cleans up markup errors,removes clutter and prettifies the markup making it easier to maintain.

Three 'flavors' of XHTML 1.0

XHTML 1.0 is specified in three 'flavors'. You specify which of thesevariants you are using by inserting a line at the beginning of the document.For example, the HTML for this document starts with a line which says that itis using XHTML 1.0 Strict. Thus, if you want to validate the document, the toolused knows which variant you are using. Each variant has its own DTD - DocumentType Definition - which sets out the rules and regulations for using HTML in asuccinct and definitive manner.

• XHTML 1.0 Strict - Use this when you want really clean structural mark-up, free of any markup associated with layout. Use this together with W3C's Cascading Style Sheet language (CSS) to get the font, color, and layout effects you want.

• XHTML 1.0 Transitional - Many people writing Web pages for the general public to access might want to use this flavor of XHTML 1.0. The idea is to take advantage of XHTML features including style sheets but nonetheless to make small adjustments to your markup for the benefit of those viewing your pages with older browsers which can't understand style sheets. These include using the body element with bgcolor, text and link attributes.

• XHTML 1.0 Frameset - Use this when you want to use Frames to partition the browser window into two or more frames.

The complete XHTML 1.0 specification is availablein English in several formats, including HTML, PostScript and PDF. See also the list of translations produced byvolunteers.

HTML 4.01

HTML 4.01 is a revision of the HTML 4.0Recommendation first released on 18th December 1997. The revision fixes minorerrors that have been found since then. The XHTML 1.0 spec relies on HTML 4.01for the meanings of XHTML elements and attributes. This allowed us to reducethe size of the XHTML 1.0 spec very considerably.

XHTMLBasic

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XHTML Basic is the second Recommendation in a series of XHTMLspecifications.

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The XHTML Basic document type includes the minimal set of modules requiredto be an XHTML Host Language document type, and in addition it includes images,forms, basic tables, and object support. It is designed for Web clients that donot support the full set of XHTML features; for example, Web clients such asmobile phones, PDAs, pagers,and settop boxes. The document type is rich enough for content authoring.

XHTML Basic is designed as a common base that may be extended. For example,an event module that is more generic than the traditional HTML 4 event systemcould be added or it could be extended by additional modules from XHTMLModularization such as the Scripting Module. The goal of XHTML Basic is toserve as a common language supported by various kinds of user agents.

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The document type definition is implemented using XHTML modules as definedin 'Modularization ofXHTML'.

The complete XHTML Basic specification isavailable in English in several formats, including HTML, plain text, PostScriptand PDF. See also the list oftranslations produced by volunteers.

XHTML Modularization

XHTML Modularization is the third Recommendation in a series ofXHTML specifications.

This Recommendation does not specify a markup language but an abstractmodularization of XHTML and an implementation of the abstraction using XMLDocument Type Definitions (DTDs) and (in version 1.1) XML Schemas. Thismodularization provides a means for subsetting and extending XHTML, a featureneeded for extending XHTML's reach onto emerging platforms.

Modularization of XHTML makes it easier to combine with markup tags forthings like vector graphics, multimedia, math, electronic commerce and more.Content providers will find it easier to produce content for a wide range ofplatforms, with better assurances as to how the content is rendered, and thatthe content is valid.

The modular design reflects the realization that a one-size-fits-allapproach no longer works in a world where browsers vary enormously in theircapabilities. A browser in a cellphone can't offer the same experience as a topof the range multimedia desktop machine. The cellphone doesn't even have thememory to load the page designed for the desktop browser.

See also XHTMLModularization for RelaxNG and an overview ofXHTML Modularization.

XHTML 1.1 -Module-based XHTML

This Recommendation defines a new XHTML document type that is based upon themodule framework and modules defined in Modularization of XHTML. The purpose ofthis document type is to serve as the basis for future extended XHTML 'family'document types, and to provide a consistent, forward-looking document typecleanly separated from the deprecated, legacy functionality of HTML 4 that wasbrought forward into the XHTML 1.0 document types.

This document type is essentially a reformulation of XHTML 1.0 Strict usingXHTML Modules. This means that many facilities available in other XHTML Familydocument types (e.g., XHTML Frames) are not available in this document type.These other facilities are available through modules defined in Modularizationof XHTML, and document authors are free to define document types based uponXHTML 1.1 that use these facilities (see Modularization of XHTML forinformation on creating new document types).

What is the difference between XHTML1.0, XHTML Basic and XHTML 1.1?

The first step was to reformulate HTML 4 in XML,resulting in XHTML 1.0. By following the HTML CompatibilityGuidelines set forth in Appendix C of the XHTML 1.0 specification, XHTML1.0 documents could be compatible with existing HTML user agents.

The next step is to modularize the elements and attributes into convenientcollections for use in documents that combine XHTML with other tag sets. Themodules are defined in Modularization ofXHTML. XHTML Basic is an example of fairlyminimal build of these modules and is targeted at mobile applications.

XHTML 1.1 is an example of a larger build of themodules, avoiding many of the presentation features. While XHTML 1.1 looks verysimilar to XHTML 1.0 Strict, it is designed to serve as the basis for futureextended XHTML Family document types, and its modular design makes it easier toadd other modules as needed or integrate itself into other markup languages. XHTML 1.1plus MathML 2.0 document type is an example of such XHTML Family documenttype.