I'm an idiot, and I don't talk with other designers/tech people enough to have my terminology straight. I meant ID, but was only using 'em for the first reason, and I didn't know about the second.
As a total side note, but you got me thinking about it: I used to have a great chart that (in the html days) had every tag and the compatibility with various browsers. Nowadays I'm using http://www.quirksmode.org/css/contents.html which is nice, but doesn't cover some of the lesser used browsers that create the PITA glitches. Where do you go for your compatibility info nowadays?