XHTML

Cleaned-up HTML

Requiring XML syntax for HTML Extensible Hypertext Markup Language (XHTML) is the W3C's effort to revamp and revitalize HTML, but it's going very slowly. The first version of XHTML, XHTML 1.0, was primarily a discussion of issues involved in the transition from the loosely-structured syntax common to HTML to the more structured approach of XML. It also provided declarations and a namespace to identify XHTML.
XHTML lets developers use XML tools, but... While XHTML 1.0 makes it easier to integrate XML and XHTML processing, and greatly simplifies the task of applying XML tools (like XSLT) to XHTML documents, nothing substantial has changed in the language or in the implementations supporting it. XHTML deployment has been subdued by the small benefits it offers and the potential incompatibilities with older browsers. XHTML does bring XML to the Web, but in a very very limited way.
Modularization The clean-up is only the first phase of the XHTML project. XHTML 1.1 redefined the XHTML vocabulary as a set of combinable modules, making it much easier to simplify XHTML for use in portable devices or reuse XHTML in other XML contexts. These should give XHTML a broader reach.
Moving Forward XHTML 2.0 isn't here yet, but there's hope that it will integrate features like XForms, a much more powerful set of tools for forms creation and processing, and some kinds of XML linking.