W3.org:

W3.org has 287 URLs listed in 163 Dmoz categories.

HTML 3.2 Specification
Historical W3C specification.
http://www.w3.org/TR/REC-html32
Top/Computers/Data_Formats/Markup_Languages/HTML/Reference/Specifications

W3C HTML Validation Service
World Wide Web Consortium validator.
http://validator.w3.org/
Top/Computers/Data_Formats/Markup_Languages/HTML/Tools/Validators_and_Lints

Overview of SGML Resources
SGML links at the W3C, meant for use in reviews of the HTML specification.
http://www.w3.org/MarkUp/SGML/
Top/Computers/Data_Formats/Markup_Languages/SGML

Comparison of SGML and XML
A Note from the W3C detailing the differences and similarities between the two markup languages.
http://www.w3.org/TR/NOTE-sgml-xml
Top/Computers/Data_Formats/Markup_Languages/SGML

Synchronized Multimedia
The official W3C page includes a brief definition and links to latest news, players, authoring tools, getting help and the other information.
http://www.w3.org/AudioVideo/
Top/Computers/Data_Formats/Markup_Languages/SMIL

Synchronized Multimedia Integration Language (SMIL 1.0)
The W3C's official technical description of the language, including the document type definition and elements within the language.
http://www.w3.org/TR/REC-smil/
Top/Computers/Data_Formats/Markup_Languages/SMIL

XHTML+SMIL Profile
Integrates a subset of the SMIL 2.0 specification with XHTML. Includes SMIL 2.0 modules providing support for animation, content control, media objects, timing and synchronization, and transition effects. (W3C Note 31 January 2002)
http://www.w3.org/TR/XHTMLplusSMIL/
Top/Computers/Data_Formats/Markup_Languages/SMIL

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.
http://www.w3.org/TR/SMIL2/
Top/Computers/Data_Formats/Markup_Languages/SMIL

W3C HTML Home Page
The W3C's HTML website, including information about HTML and XHTML.
http://www.w3.org/MarkUp/
Top/Computers/Data_Formats/Markup_Languages/XHTML

World Wide Web Consortium (w3c): XHTML Modularization
7 sections: introduction; terminology; module construction; DTD construction; module examples; mark-up language examples; usage examples.
http://www.w3.org/MarkUp/Guide/xhtml-m12n-tutorial/
Top/Computers/Data_Formats/Markup_Languages/XHTML/FAQs,_Help,_and_Tutorials