Why don't they document this shit? Anywhere?
For some reason, the best way to get a fixed-width sidebar that doesn't break on at least one common browser (*cough* IE) is to use negative margins and floats. Strangely, positive margins break things in some browsers but negative margins don't!
Why? Negative margins allow overlap into that element's space. And I'm damned if I know why that solves the problem, but it seems to, and hopefully I can fix all my style woes this way, since horizontal positioning and margins are pretty much the only major issues I've got.
I ... just ... can't ... believe ... it's ... that ... simple. But I bloody well hope it is. It's been three weeks since the launch and my site still looks badly broken in half the browsers I test it in.
Props to the cssDiscuss wiki, the most awesome CSS reference ever. I spent an hour today RTFMing and the wiki still helped me more, and more quickly.