What is XHTML? XHTML stands for EXtensible HyperText Markup Language. XHTML - Introduction - XHTML stands for EXtensible HyperText Markup. XHTML is almost identical to HTML but it is stricter than HTML. It contains well written, well thought and well explained computer science and. Extensible HyperText Markup Language (XHTML) is part of the family of XML markup languages. What is XHTML? XHTML is a mix of HTML (the HyperText Markup Language) and XML (eXtensible Markup Language).