One of the privileges I have experienced in my professional life is having worked with Jeremy Keith of Clearleft. I learned a lot from the author and web developer from our interactions and by studying his code.
One example is a thumbnails carousel he designed. If the browser does not have JS support, the thumbnails are arranged in a grid. If the client does support JS, he wraps-up the thumbnails in a single row inside a horizontally-scrolling carousel, so they can be browsed left to right. The carousel presentation of the thumbnails requires a different set of CSS rules, which override the vanilla styles.
Any site I’m involved in will be following suit.
- Page-specific styles. I see CSS as a way to define general styles that affect large portions of a site (using classes). If you want to do something really, really special on a particular page, just put it inside the handful of elements that want to be different as inline styles. (ID selectors are a dying breed.) There’s no need to define rules in a global style sheet if they’re not really global. If the page has a lot of inline styles, and they are making the markup unmanageable, you can always extract the styles into a page-specific stylesheet. One extra HTTP request on one page (or a handful of pages) won’t kill you or your users.