Different Types of DOCTYPE
http://www.w3.org/QA/2002/04/valid-dtd-list.html
XHTML 1.0 Strict Cheat Sheet
http://www.w3.org/2010/04/xhtml10-strict.html
HTML Document Structure
http://www.w3.org/TR/html401/struct/global.html
Markup Validation Tool
http://validator.w3.org/
The Future of HTML
http://dev.w3.org/html5/spec/Overview.html