eXtensible HyperText Markup Language (XHTML) is a reformulation of HTML 4.0 as an application of XML 1.0 (as opposed to HTML, which is an application of SGML).

• The 'application/xhtml+xml' Media Type - (IETF RFC 3236) defines the 'application/xhtml+xml' MIME media type for XHTML based markup languages, Mark A. Baker and Peter Stark, January 2002
• Building XHTML Modules - (W3C Working Draft as of 2002-08-20)
• Modularization of XHTML - (W3C Recommendation) abstract modularization of XHTML and an implementation of the abstraction using XML Document Type Definitions (DTDs), which provides a means for subsetting and extending XHTML
• Modularization of XHTML in XML Schema - (W3C Working Draft as of 2002-08-20) set of XML Schema modules for XHTML, and a framework for extending and modifying XHTML
• Ruby Annotation - (W3C Recommendation) extends XHTML to support ruby text, which is a short run of text alongside base text typically used in East Asian documents to indicate pronunciation or annotation
• Implementing the Ruby Module - (W3C Note as of 2001-07-02) describes sample module implementations of Ruby Annotation's abstract definition of ruby markup in several schemas: DTD, RELAX, TREX, and XML Schema
• An XHTML + MathML + SVG Profile - (W3C Working Draft as of 2002-08-20) enables mixing XHTML, MathML and SVG in the same document using XML namespaces mechanism, while allowing validation of such a mixed-namespace document
• eXtensible HyperText Markup Language (XHTML), Version 1.0 - (W3C Recommendation) several errata are addressed in a Second Edition
• XHTML 1.0 in XML Schema - (W3C Note as of 2002-09-11) provides informative XML schemas corresponding to the XHTML 1.0 Strict, Transitional, and Frameset DTDs
• XHTML 1.1 - Module-based XHTML - (W3C Recommendation) reformulation of XHTML 1.0 Strict based on XHTML modules, including the ruby module
• XHTML 2.0 - (W3C Working Draft as of 2002-08-07) specifies the XHTML 2.0 Markup Language and a variety of XHTML-conforming modules that support that language
• XHTML Basic - (W3C Recommendation) designed for Web clients that do not support the full set of XHTML features
• XHTML Document Profile Requirements - (W3C Working Draft as of 2001-01-03)
• XHTML Media Types - (W3C Note as of 2002-08-07) summarizes the best current practice for using various Internet media types for serving various XHTML Family documents
• XHTML+SMIL Profile - (W3C Notes as of 2002-08-07) includes modules for animation, content control, media objects, timing and synchronization, time manipulations, and transition effects