I just finished making the necessary changes to make the entire site validate. This site does use CSS3, which isn't even official yet. However, the features that I've used are already supported by the NGLayout, a.k.a. Gecko, engine, which is used by many browsers such as Mozilla, Firefox, and Epiphany, and the site is still very usable without support for those features. The CSS validator in CSS3 mode still has a lot of bugs and produces warnings for code that's perfectly fine, although it still agrees that my site uses valid CSS3.