http://www.hitbox.com/cgi-bin/page.cgi?tools/doc
This one is a lot more useful, IMO. Just make sure you get a "perfect" rating, and you know your site will do. (it also checks spelling, meta tags, broken links and load time, aside from html syntax)
And no, the surfer doesn't give a flying fuck about W3C standards,
as long as he can see the page in whatever browser he's using.