I definitely agree that that is the best site for learning HTML. I just want to share some tips on good pages when you need a bit deeper knowledge on a certain HTML tag. Some good HTML reference pages are:
W3C's index of HTML4 elements
WDG's HTML 4 Element list
Glubase's wiki XHTML/HTML command reference
Theese are pretty good CSS references:
W3School's CSS2 reference
Glubase's wiki CSS2 reference
