The W3C has released XHTML as `a reformulation of HTML 4 in XML 1.0' . This specification defines HTML as an XML application, and provides three DTDs corresponding to the ones defined by HTML 4.0. The semantics of the elements and their attributes are as defined in the W3C Recommendation for HTML 4.0. These semantics provide the foundation for future extensibility of XHTML. Compatibility with existing HTML user agents is possible by following a small
set of guidelines.