My favourite sites:
Index DOT Html: Alphabetic Sorted HTML Tag Listing
bruce-hamilton.com: Server Side Includes (SSI) Tutorial, Apache: Tutorial/Introduction to SSI
HTML entities listed: http://www.bbsinc.com/iso8859.html
W3C: HTML Validator
CSS: Dave Raggett's Introduction, RichInStyle
I hope you find these useful.

-ZeroOne