HTML
Books, Chats and Forums, Reference, Resources, Tools, Tutorials, All My FAQs Wiki: HTML FAQs, An Introduction to HTML Email, Drew's Design's, Extreme HTML Optimization - WebReference, HTML and JS - Dream Quest 4U, HTML FAQ Knowledge Base - IRT, HTML Q&A - Allexperts, Latte, Life of a Website, Navigator and Explorer Comparison: HTML Tags
SGML
Applied Languages, Companies, Groups, HyTime, References and Standards, Software, Style Sheets, A Gentle Introduction to SGML, Comparison of SGML and XML, Graphic Communications Association, Overview of SGML Resources, SGML for NT, SGML on the Web, SGML: A Gentle Introduction, SGMLtools-Lite, SP, The SGML/XML Web Page
SMIL
A walk in the Forest, Confluent Technologies, GRiNS, HELIO, JustSMIL, Learn SMIL with a SMIL Presentation, NIST S2M2 Project, PerlySMIL, RealPix: The SMIL Graphics Format, Schmunzel
XHTML
News and Media, References, Specifications, Tools, Tutorials, HTML Home Page, New Architect, NYPL Styleguide, The case for XHTML, The HTML Writers Guild, Web Developer Foundations: Using XHTML, XDG: XHTML Design Guide, XHTML :: How to succeed, XHTML is the Most Important XML Vocabulary, XHTML Web Design for Beginners
XML
Addressing and Querying, Applications, Consultants, Document Types, Encoding, Forms, Linking, Namespaces, News and Media, People
Electronic Documents Markup
A brief explanation of what markup languages are and where they come from.
Project Xanadu
The original hypertext project. Includes a history of the project and gives credit to the early developers of markup languages.
whatis.com: markup
A short definition.
YAML Ain't Markup Language (YAML)
A straightforward machine parsable data serialization format designed for human readability and interaction with scripting languages such as Perl and Python.