It must validate against one of the three DTDs found in Appendix A.
The root element of the document must be <html>.
The root element of the document must designate the XHTML namespace using the xmlns attribute [XMLNAMES]. The namespace for XHTML is defined to be http://www.w3.org/1999/xhtml.
There must be a DOCTYPE declaration in the document prior to the root element. The public identifier included in the DOCTYPE declaration must reference one of the three DTDs found in Appendix A using the respective Formal Public Identifier. The system identifier may be changed to reflect local system conventions.