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.
Then again when many people use things like MS Front Page to design their sites...of course MS HTML authoring programs will favor their browser over a competitor...
In reality it seems one needs both browsers today due to what one can run into on various sites. There is one big complaint I have with Mozilla and Netscape 6.x (which I never had with Netscape 4.7x) but this is with Messenger. It comes down to saving one's store of email. Back it up, do a format and clean install, then reinstall the browser, one's email profile with all the former messages (all organized in folders they were previously in) is no longer there. In Netscape 4.7x it was, I consider this handling in the mail client to be a step backwards, and major reason I switched to OE.
Unlike the old time AOL users (of which I was never one) some of us do want to save our mail messages permanently or semi-permanently, and be free to completely wipe and reinstall our system (if needed) without any of this being lost accross new installs. Takes too much control over this out of the users hands for my own tastes