This specification defines XHTML 1.0, a reformulation of HTML 4 as an
XML 1.0 application, and three DTDs corresponding to the ones defined
by HTML 4.

The semantics of the elements and their attributes are defined in the
W3C Recommendation for HTML 4.

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.