I was just playing around with Firebug and editing Gmail's CSS file, and I wanted to edit a button, but the div ID for that button was :rj. I am fairly certain that CSS does not allow colons in —and especially starting as—ID and class names. So my guess is this is some advanced trickery. I'm not sure if it's consistent like this for each user, but FWIW, the ID was for the "Search Mail" button at the top of the page.
Does anyone know what they are doing and how they are doing it?