XHTML 2.0 is a markup language intended for rich, portable web-based applications. While the ancestry of XHTML 2.0 comes from HTML 4, XHTML 1.0, and XHTML 1.1, it is not intended to be backward compatible with its earlier versions. Application developers familiar with its earlier ancestors will be comfortable working with XHTML 2.0.
XHTML 2 is a member of the XHTML Family of markup languages. It is an XHTML Host Language as defined in Modularization of XHTML. As such, it is made up of a set of XHTML Modules that together describe the elements and attributes of the language, and their content model. XHTML 2.0 updates many of the modules defined in Modularization of XHTML, and includes the updated versions of all those modules and their semantics. XHTML 2.0 also uses modules from Ruby, XML Events, and XForms.