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The W3C Recommendation is XHTML which is ‘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.* (Strict, Transitional, and Frameset).
The semantics of the elements and their attributes are as defined in the W3C Recommendation for HTML 4. These semantics were intended to provide the foundation for future extensibility of XHTML. Compatibility with existing HTML browsers is possible by following a small set of guidelines (see the W3C site).