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This section is informative.
XHTML 2 is a markup language intended for rich, portable web-based applications. While the ancestry of XHTML 2 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. Appendix A describes the ways in which XHTML 2 differs from previous versions and what application developers need to know to convert existing applications to XHTML 2.
XHTML 2 is a member of the XHTML Family of markup languages. It is an XHTML Host Language as defined in XHTML Modularization. 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 updates many of the modules defined in XHTML Modularization 1.0 [XHTMLMOD], and includes the updated versions of all those modules and their semantics. XHTML 2 also uses modules from Ruby [RUBY], XML Events [XMLEVENTS], and XForms [XFORMS].
The modules defined in this specification are largely extensions of the modules defined in XHTML Modularization 1.0. This specification also defines the semantics of the modules it includes. So, that means that unlike earlier versions of XHTML that relied upon the semantics defined in HTML 4, all of the semantics for XHTML 2 are defined either in this specification or in the specifications that it normatively references.
Even though the XHTML 2 modules are defined in this specification, they are available for use in other XHTML family markup languages. Over time, it is possible that the modules defined in this specification will migrate into the XHTML Modularization specification.