XHTML


Extensible HyperText Markup Language is part of the family of XML markup languages which mirrors or extends versions of the widely used HyperText Markup Language, the language in which Web pages are formulated.
While HTML, prior to HTML5, was defined as an application of Standard Generalized Markup Language, a flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML. XHTML documents are well-formed and may therefore be parsed using standard XML parsers, unlike HTML, which requires a lenient, HTML-specific parser.
XHTML 1.0 became a World Wide Web Consortium recommendation on 26 January 2000. XHTML 1.1 became a W3C recommendation on 31 May 2001. XHTML is now referred to as "the XML syntax for HTML" and being developed as an XML adaptation of the HTML living standard.

Overview

XHTML 1.0 was "a reformulation of the three HTML 4 document types as applications of XML 1.0". The World Wide Web Consortium also simultaneously maintained the HTML 4.01 Recommendation. In the XHTML 1.0 Recommendation document, as published and revised in August 2002, the W3C commented that "The XHTML family is the next step in the evolution of the Internet. By migrating to XHTML today, content developers can enter the XML world with all of its attendant benefits, while remaining confident in their content's backward and future compatibility."
However, in 2005, the Web Hypertext Application Technology Working Group formed, independently of the W3C, to work on advancing ordinary HTML not based on XHTML. The WHATWG eventually began working on a standard that supported both XML and non-XML serializations, HTML5, in parallel to W3C standards such as XHTML 2.0. In 2007, the W3C's HTML working group voted to officially recognize HTML5 and work on it as the next generation HTML standard. In 2009, the W3C allowed the XHTML 2.0 Working Group's charter to expire, acknowledging that HTML5 would be the sole next-generation HTML standard, including both XML and non-XML serializations. Of the two serializations, the W3C suggests that most authors use the HTML syntax, rather than the XHTML syntax.
The W3C recommendations of both XHTML 1.0 and XHTML 1.1 were retired on 27 March 2018, along with HTML 4.0, HTML 4.01, and HTML5.

Motivation

XHTML was developed to make HTML more extensible and increase interoperability with other data formats. In addition, browsers were forgiving of errors in HTML, and most websites were displayed despite technical errors in the markup; XHTML introduced stricter error handling. HTML 4 was ostensibly an application of Standard Generalized Markup Language ; however the specification for SGML was complex, and neither web browsers nor the HTML 4 Recommendation were fully conformant to it. The XML standard, approved in 1998, provided a simpler data format closer in simplicity to HTML 4. By shifting to an XML format, it was hoped HTML would become compatible with common XML tools; servers and proxies would be able to transform content, as necessary, for constrained devices such as mobile phones.
By using namespaces, XHTML documents could provide extensibility by including fragments from other XML-based languages such as Scalable Vector Graphics and MathML. Finally, the renewed work would provide an opportunity to divide HTML into reusable components and clean up untidy parts of the language.

Relationship to HTML

There are various differences between XHTML and HTML. The Document Object Model is a tree structure that represents the page internally in applications, and XHTML and HTML are two different ways of representing that in markup. Both are less expressive than the DOM – for example, "--" may be placed in comments in the DOM, but cannot be represented in a comment in either XHTML or HTML – and generally, XHTML's XML syntax is more expressive than HTML. XHTML uses an XML syntax, while HTML uses a pseudo-SGML syntax. Because the expressible contents of the DOM syntax are slightly different, there are some changes in actual behavior between the two models. Syntax differences, however, can be overcome by implementing an alternate translational framework within the markup.
First, there are some differences in syntax:
  • Broadly, the XML rules require that every element be closed, either with a separate closing tag or by using the self-closing syntax, while HTML syntax permits some elements to be unclosed because either they are always empty or their end can be determined implicitly.
  • XML is case-sensitive for element and attribute names, while HTML is not.
  • Some shorthand features in HTML are omitted in XML, such as attribute minimization, where attribute values or their quotes may be omitted ; element minimization may be used to remove elements entirely ; and the rarely used SGML syntax for element minimization, which most browsers do not implement.
  • There are numerous other technical requirements surrounding namespaces and precise parsing of whitespace and certain characters and elements. The exact parsing of HTML in practice has been undefined until recently; see the HTML5 specification for full details, or the working summary.
In addition to the syntactical differences, there are some behavioral differences, mostly arising from the underlying differences in serialization. For example:
  • Behavior on parse errors differs. A fatal parse error in XML causes document processing to be aborted.
  • Most content requiring namespaces will not work in HTML, except the built-in support for SVG and MathML in the HTML5 parser along with certain magic prefixes such as xlink.
  • JavaScript processing is different in XHTML, with minor changes in case sensitivity to some functions, and further precautions to restrict processing to well-formed content. Scripts must not use the method; it is not available for XHTML. The innerHTML property is available, but will not insert non-well-formed content. On the other hand, it can be used to insert well-formed namespaced content into XHTML.
  • Cascading Style Sheets are also applied differently. Due to XHTML's case-sensitivity, all CSS selectors become case-sensitive for XHTML documents. Some CSS properties, such as backgrounds, set on the element in HTML are 'inherited upwards' into the element; this appears not to be the case for XHTML.

    Adoption

The similarities between HTML 4.01 and XHTML 1.0 led many websites and content management systems to adopt the initial W3C XHTML 1.0 Recommendation. To aid authors in the transition, the W3C provided guidance on how to publish XHTML 1.0 documents in an HTML-compatible manner, and serve them to browsers that were not designed for XHTML.
Such "HTML-compatible" content is sent using the HTML media type rather than the official Internet media type for XHTML. When measuring the adoption of XHTML to that of regular HTML, therefore, it is important to distinguish whether it is media type usage or actual document contents that are being compared.
Most web browsers have mature support for all of the possible XHTML media types. The notable exception is Internet Explorer versions 8 and earlier by Microsoft; rather than rendering application/xhtml+xml content, a dialog box invites the user to save the content to disk instead. Both Internet Explorer 7 and Internet Explorer 8 exhibit this behavior. Microsoft developer Chris Wilson explained in 2005 that IE7's priorities were improved browser security and CSS support, and that proper XHTML support would be difficult to graft onto IE's compatibility-oriented HTML parser; however, Microsoft added support for true XHTML in IE9.
As long as support is not widespread, most web developers avoid using XHTML that is not HTML-compatible, so advantages of XML such as namespaces, faster parsing, and smaller-footprint browsers do not benefit the user.

Criticism

In the early 2000s, some Web developers began to question why Web authors ever made the leap into authoring in XHTML. Others countered that the problems ascribed to the use of XHTML could mostly be attributed to two main sources: the production of invalid XHTML documents by some Web authors and the lack of support for XHTML built into Internet Explorer 6. They went on to describe the benefits of XML-based Web documents regarding searching, indexing, and parsing as well as future-proofing the Web itself.
In October 2006, HTML inventor and W3C chair Tim Berners-Lee, introducing a major W3C effort to develop a new HTML specification, posted in his blog that "he attempt to get the world to switch to XML... all at once didn't work. The large HTML-generating public did not move... Some large communities did shift and are enjoying the fruits of well-formed systems... The plan is to charter a completely new HTML group." The current HTML5 working draft says "special attention has been given to defining clear conformance criteria for user agents in an effort to improve interoperability... while at the same time updating the HTML specifications to address issues raised in the past few years." Ian Hickson, editor of the HTML5 specification criticizing the improper use of XHTML in 2002, is a member of the group developing this specification and is listed as one of the co-editors of the current working draft.
Simon Pieters researched the XML-compliance of mobile browsers and concluded "the claim that XHTML would be needed for mobile devices is simply a myth".

Versions of XHTML

XHTML 1.0

December 1998 saw the publication of a W3C Working Draft entitled Reformulating HTML in XML. This introduced Voyager, the codename for a new markup language based on HTML 4, but adhering to the stricter syntax rules of XML. By February 1999 the name of the specification had changed to XHTML 1.0: The Extensible HyperText Markup Language, and in January 2000 it was officially adopted as a W3C Recommendation. There are three formal Document Type Definitions for XHTML 1.0, corresponding to the three different versions of HTML 4.01:
  • XHTML 1.0 Strict is the XML equivalent to strict HTML 4.01, and includes elements and attributes that have not been marked deprecated in the HTML 4.01 specification., XHTML 1.0 Strict is the document type used for the homepage of the website of the World Wide Web Consortium.
  • XHTML 1.0 Transitional is the XML equivalent of HTML 4.01 Transitional, and includes the presentational elements excluded from the strict version.
  • XHTML 1.0 Frameset is the XML equivalent of HTML 4.01 Frameset, and allows for the definition of frameset documents—a common Web feature in the late 1990s.
The second edition of XHTML 1.0 became a W3C Recommendation in August 2002.