May 26, 2018
W3C’s XHTML DTD
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.