There’s a new feature in the WebKit nightlies: media queries. Media queries allow you to “query” the user agent for some of it’s specifics right from within the CSS, and match your CSS to that. I have a media queries page in my CSS3 section, which shows some examples of it’s use.
One of the cool things about this, next to it being one of my favourite parts of the CSS3 spec, is that it was created by one of the Nokia guys. I love the fact that we’re seeing results of them joining the WebKit community so soon.