Advice if you build websites: Use the W3C HTML *and* CSS validator. Not because I want to tell you that "you're bad if your HTML isn't valid", but because it'll help you find trivial bugs that can make your life hard.
Especially also use the CSS validator. I found 3 issues that made things not work as they should in recent days. I had a bug where it took me a while to figure out why it didn't work til I found some stackoverflow post, and the css validator would've told me right away.