Really advanced CSS
There is much more to CSS than just styling borders and text. Unfortunately much of the really exciting stuff is not supported by many, if any, web browsers yet.
Someone who has experimented a lot with advanced CSS is Literary Moose. The Nemesis Project has published Advanced CSS Ornamentation, a three-part article in which Literary Moose explores the limits of what CSS can do. This long article shows some things that will be possible in the future, provided that browser vendors keep improving CSS support.
So when is the future? Hard to tell. My guess is around 2008, if ever, for Internet Explorer users. Much earlier for people who use browsers like Mozilla or Safari. Even sooner for those who use the latest version of Opera.