xHTML/CSS has some common flaws that are uncompatible in some browsers eg IE6. IE7 renders alot more like FF than IE6, so techically you can only write for FF, which supports more CSS yes, but if you were a good coder you would be able to code flawlessly for all browsers by habit, knowing what to avoid.

Also, techincally if cross-browser compatibility is not stated while you discus terms of the job its not technically a requirement, but is expected of all coders who are worth thier weight.