An XML markup language is defined by a DTD. DTDs specify strict syntax of the XML markup language. XHTML is HTML that has a DTD that defines it, thus XHTML is an XML markup language. Basically, XHTML is HTML with strict syntax rules. These are the "incantations" that are needed for XHTML Strict: This is optional. Without it the document is only "tentatively" valid. validator.w3.org