I just finished going through the W3Schools' HTML and XHTML tutorials. HTML is one of those things that I know, but have never really been a master in. I guess that has to do with the fact that the only time I was actually taught html was in high school. At university we did get a few snippets here and there (I remember Ulrich teaching us CSS) but in general we didn't learn it.
Going through the tutorials was interesting if only because it gave you a nice idea what can be done with just pure (x)html. There are a lot of things I thought were quite new and novel, part of the web 2.0 thing, that I now realise were around for a while. I also really like the reference section and the link to the W3C validators. I'm gonna try to validate any websites I write against those validators from now on.