- arbitrary extensions to the set of defined elements; and
- documents to be more easily understood and manipulated as DOM hierarchies by programs.
XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C Recommendation on May 31, 2001. XHTML5 is undergoing development as of June 2010, as a variant syntax of HTML5. (XHTML2 was being developed in parallel with HTML5, but the W3C abandoned it; the version-number 5 prevents confusion between the two standards and synchronizes the versioning of the two syntaxes.)