CSS 2.1 selectors offer a lot of functionality which allow to create clutter-free html by getting rid of all these .class and #id tags we currently often apply to every 2nd element
There’s a lack of support especially in the various incarnations of Internet Explorer so far, but most other modern browsers geht it right, so learning about using these selectors seems to be a worthfull idea.
And apart from that: on the one hand there’re some Javascript extensions which teach Internet Explorer to “understand” these selectors, and on the other hand is IE7 on the horizon too.