Hi,
http://www.w3.org/MarkUp/
http://validator.w3.org/
I'm all for teaching students how to look up the answer, but in this case I also recommend they read http://www.alistapart.com/articles/readspec/ first so they have a better chance of understanding the answers :)
I've never found a "standards-compliant html for beginners" resource that I'm 100% happy with, although W3Schools isn't too bad. I also direct people to A List Apart, as has already been suggested; although some people find it a bit daunting depending on the issue they read first :)
As a student improves they can probably get something out of the links at http://zeldman.com/externals/#cssmarkup as well.
cheers,
Ben
--
--- <http://www.200ok.com.au/>
--- The future has arrived; it's just not
--- evenly distributed. - William Gibson
