I came across a totally annoying problem in Hotmail today. I had an H2 immediately followed by an H3, with no margin between. No amount of !importants could deal with it. So in Firebug I examined the code, and Hotmail has a special declaration that I couldn't make CM overwrite: H2 + H3.
So I had to rename my H2 and H3, and then it worked perfectly.
Just thought I would post this in here incase someone else comes across it, or I need reminding how to fix it :>