Dani Iswara pointed out a new (it's new for me, but perhaps not for you) validator, called Validome. Basically it has the same purpose of W3's Validator, but i don't know if it share the same engine or not. He said that the result of his testing was different for the same site, so i'm quite curious about that and decided to have a look on my personal website, because it's stated valid as XHTML 1.0 Transitional based on W3's Validator. I checked my website there and it appears to be valid as well. That's a good sign. I made the website manually, so it's easier to get it valid, because you can change it whenever you need to and i'm the one who knows about the structure/layout of the code, so it's easy to do that. That's one of the advantages of building your own web site. But, nowadays, many CMS producs are valid as well by default, so it's getting easier to have a valid web page.
Validome doesn't offer CSS validator, but they offer some other validation tool, such as XML, DTD-Schema, RSS/Atom, and Google Sitemap validation. If your site is based on XML format, perhaps you can use this feature to test yours. It's a great site.
Credit to Dani