Magnificently ingenious way to use CSS to make informed guesses about what sites might be in a user’s browser history. Using this trick you can, for example, show a “share on facebook” link for those people who use facebook, but not show “share on $other_random_site”. I hate those rows of “digg this!”-type buttons. This could help solve that, though there’s a bit of my user-privacy-sense tingling right now.

So clever.