What is XHTML?

The Extensible HyperText Markup Language (XHTML): the successor of HTML as a family of XML modules

XHTML is not the name of a single, monolithic markup language, but the name of a family of document types (called XHTML Family) which collectively form this markup language

The XHTML namespace is identified by the URI http://www.w3.org/1999/xhtml which is shared across XHTML Family members