@pyroJun 27.2004 — #You don't. CSS is not a scripting language.
However, you can often use browser's ability (or in IE's case, disability :rolleyes? to understand certain CSS selectors, properties, etc. to send certain styles to certain browsers. The more you can avoid doing this, the better, though.
@Ben_RogersJun 28.2004 — #Well, depending on the type of conditions you want, you could print out a style sheet with [url=http://www.php.net/]PHP[/url] or some other server side language. Also CSS3 offers some dynamic pseudo classes, but that's only supported in actual browsers.