CSS - and all that can be done with it - shows enormous promise. Unfortunately, it's not too well supported. A lot of CSS can be used safely, with a very low possibility of screwing things up. A lot of it can't. With time and experience, you'll probably find out which CSS properties you should use and which you shouldn't.
Unfortunately, few of us really have time to work this sort of thing out (well, I do, but only when no-one realises I'm not working), and there are times when you'd just rather forgo the experience of worrying about browsers in order to, well, get experience. If you get what I mean.
Fortunately, since the dawn of time a few brave souls have put together documents and charts describing the support (or lack thereof) many browsers have for CSS. Where we find them, you'll find them - here.