HTML5: W3C Candidate Recommendation specification for a vocabulary and associated APIs for HTML and XHTML

Mathematical Markup Language (MathML): Official page from the World Wide Web Consortium. Includes the MathML specification, links to related software, and other resources

Synchronized Multimedia: The official W3C page includes a brief definition and links to latest news, players, authoring tools, getting help and the other information

Synchronized Multimedia Integration Language (SMIL 2.0): XML-based language that allows authors to write interactive multimedia presentations, and allows reuse of SMIL syntax and semantics in other XML-based languages, in particular those who need to represent timing and synchronization

The SGML/XML Web Page: Database containing over 2000 documents pertaining to SGML, XML, and related standards. Features a bibliography for SGML, vendors and service providers, software reviews, mailing lists, user groups, and projects

W3C HTML Home Page: The W3C's HTML website, including information about HTML and XHTML

Web Hypertext Application Technology Working Group (WHATWG): A community of people interested in the development of HTML and APIs needed for Web applications. Its main focus is HTML5. Includes wiki, forums, FAQ and specifications

XHTML 1.1 - Module-based XHTML: The current W3C XHTML specification. Includes definitions and documentation for XHTML