This is quite difficult. There are a great many tutorials around, but
most aren't very good. The hard part is avoiding the bad ones!
I like books, so I like "Head First HTML with CSS & XHTML" from
O'Reilly
<
http://www.amazon.com/exec/obidos/ASIN/059610197X/codesmiths-20>
It's the best tutorial I know, and it teaches good "style" as well
(this is very rare).
I also like Lie & Bos' "Cascading Style Sheets"
<
http://www.amazon.com/exec/obidos/ASIN/0321193121/codesmiths-20>
This is a good CSS tutorial, accessible to HTML beginners too, and
remains a useful desktop reference for CSS afterwards.
For web sites, <
http://htmldog.com> is the best I've seen, even though
it's nothing like the detail of the books.
<
http://www.w3.org/TR/html4/cover.html> is also essential, as it's the
"horse's mouth" for the specification. Not easy reading though!
Avoid W3Schools.