Sams Teach Yourself Web Publishing with HTML and CSS in One Hour a Day (5th Edition)

HTML Specification and Info

http://www.w3.org/MarkUp/

XHTML 1.0

http://www.w3.org/TR/xhtml1/

HTML 4.01

http://www.w3.org/TR/html4/

The HTTP specification (as defined in 1992)

http://www.w3.org/hypertext/WWW/Protocols/HTTP/HTTP2.html

Cascading Style Sheets

http://www.w3.org/Style/CSS/

Information about HTTP

http://www.w3.org/Protocols/

Pointers to URL, URN, and URI information and specifications

http://www.w3.org/Addressing/