Incorrect mime type for CSS files. Why am I linking this, you ask? Well, we spent half a day trying to figure out why our valid xhtml document was refusing to render the stylesheet in Netscape 7. It just wouldn't play nicely. The problem? Our web server happens to be iPlanet, which happens to have an incorrect mime type set for .css files. No wonder we were breaking our heads trying to figure it out - it wasn't the browser or the document; it was the server. Which is crazy, really.
So if your page breaks in Netscape 7 when you add the correct DOCTYPE, go check your server config. It does effect Apache too. Of course, you should remember that searching for help on Netscape 7 is waste of fucking time. Just look up Mozilla, or perhaps Gecko, or perhaps look up jobs that don't involve working with computers.