XHTML

Extensible HyperText Markup Language (XHTML) is part of the family of XML markup languages. It 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. 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 still 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.