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.