I know this is probably minor to most people, but it's really one of the fine details that I'd appreciate. These are just the errors I get from global.css, there are more in skin.css (mostly filter errors), but you get the idea.
Lines 6,1575,1795: Invalid CSS comment with // instead of /* */
Lines 43,132,969,973,977: IE only behaviour served to all browsers
Line 52: Semicolon missing
Lines 2642,2647,2648,2653: Comment inside CSS with "#".
Line 2469: Invalid CSS property resize (don't know that one, could be MSIE only)