XHTML (Extensible HyperText Markup Language)
XHTML is the new, cleaner and more precise version of HTML, made by combining HTML 4.01 and XML, taking the elements from HTML and the syntax from XML. It is almost identical to HTML 4.01, so there is not a lot of changes. XHTML was created to replace HTML completely.
W3C recommends using XHTML since January 2000. That means it is stable and checked by people from W3C organisation.