The w3schools site seems to be very good. I went through there xml tutorial it was very easy to follow and a very good starting point... although I havent done anything with xml since I am sure there xhtml and css tutorials are just as good. From what I remeber they have a tag reference and explanations of many tags so thats good.

Another site http://webmonkey.wired.com/webmonkey...g/html_basics/
I am not sure how much xhtml and css this site has but a friend that is learning html said he is finding it useful.