XHTML 1.0 document type definitions

This provides the DTDs of the Second Edition of 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.