Richard Allsebrook writes about sniffing the user agent to serve browser-specific style sheets (via css vault).

FWIW, my opinion is that in the long run this is more trouble than it’s worth since it requires maintaining multiple style sheets, but if you wish to use this technique, TopStyle Pro’s “CSS Export” may provide some help. This feature takes a style sheet and creates a “downgraded” version by stripping out styles problematic to a specific browser.