The Extensible Hypertext Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML, but also conforms to XML syntax.

HTML is the antecedent technology to XHTML. The changes from HTML to first-generation XHTML 1.0 are minor and are mainly to achieve conformance with XML. The most important change is the requirement that the document must be well-formed and that all elements must be explicitly closed as required in XML. In XML, all elements and attribute names are case-sensitive, so the XHTML approach has been to define all tag names to be lowercase.