Do you think having valid xHTML and CSS on your forums skin is something good to have? I have just been learning html and css, and I would say yes. Like other websites, a forum should be coded validly too.
I think websites and forums should be treated the same when coding. I am still working on getting AU valid xHTML.
I am guessing you have firebug or webdeveloper? It really depends on the community, you really do want a valid page if your forum is webmaster orientated.
Yer thats IE right? Teeehee. Yep, valid HTML/XHTML is good to have, valid CSS im my opinion isn't that important.