HTML: As long as you do no big mistakes the w3c html validator reports no errors. Test at:
You can also check out some other pages, most webpages are not w3c conform - Happy pageWinder user ;)

CSS: Unfortually the css checker report some errors for the used addons genericon and lightbox. I did not check it in deep, but looks like both addons only try to make all browser happy so it's not 100% w3c conform.