Extensible HyperText Markup Language (XHTML) is part of the family of XML markup languages which mirrors or extends versions of the widely used HyperText Markup Language (HTML), the language in which Web pages are formulated. While HTML, prior to HTML5, was defined as an application of Standard Generalized Markup Language (SGML), 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 (W3C) 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 (W3C) 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 (WHATWG) 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 (SGML); 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 (XHTML Modularization) and clean up untidy parts of the language.
Relationship to HTML There are various differences between XHTML and HTML. The Document Object Model (DOM) 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 (for example, arbitrary namespaces are not allowed in HTML). XHTML uses an XML syntax, while HTML uses a pseudo-SGML syntax (officially SGML for HTML 4 and under, but never in practice, and standardized away from SGML in HTML5). 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 (e.g. </div>) or by using the self-closing syntax (e.g. <br/>), while HTML syntax permits some elements to be unclosed because either they are always empty (e.g. <input>) or their end can be determined implicitly ("omissibility", e.g. <p>). XML is case-sensitive for element and attribute names, while HTML is not. Some shorthand features in HTML are omitted in XML, such as (1) attribute minimization, where attribute values or their quotes may be omitted (e.g. <option selected> or <option selected=selected>, while in XML this must be expressed as <option selected="selected">); (2) element minimization may be used to remove elements entirely (such as <tbody> inferred in a table if not given); and (3) the rarely used SGML syntax for element minimization ("shorttag"), 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 ([HTML5]) for full details, or the working summary (HTML vs. XHTML). In addition to the syntactical differences, there are some behavioral differences, mostly arising from the underlying differences in serialization. For example:
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