I have to say that IE is NOT the most compatible browser with regards to standards. The don't properly implement some tags of CSS1 (let alone CSS2) and they handle other things wrong. They look like the most compatible because web designers can sometimes fudge web pages to work with specific browsers. Check out this PNG Demo
with IE and tell me what you see.
Ars Technica put up a review of Mozilla. Follow this link
to read the review.