Digest this site.
If you can "get" most of it, and stick to your guns when people suggest Stupid Web Tricks, you'll be light years ahead of most designers/developers out there. Most designers come to it late and all they can seem to do is argue and justify their designs, and plod forward committing web crimes. Start there, you'll be ahead of the game.
Second would be this very site. Understand why you would choose XHTML over HTML. It's all here.