So it was with great joy that I found this site today: W3Fools. They detail all the reasons that I don't recommend W3Schools, and include better references. No, they don't include this site, but that's okay. I'd rather you get your HTML, CSS, and XML information from the reputable sites they list than from W3Schools, even if you're not getting it from me.
And to clarify, just because they are named W3Schools does not mean they are associated in any way with the web standards organization W3C. Anyone can buy a domain with W3 in the URL—but only sites on the w3.org domain are legitimate W3C sites. Don't be fooled.