We used to develop under the "conform to the lowest common denominator" standard, and possible add bells and whistles for individual browsers. Now we develop the other way around.
I still think Opera is the best choice for a developmental browser (see my other comment). Once I get a page working correctly there, I test on all the others, and tweek it as needed. Usually, no matter what I do to fix another browser's idiosyncrasies, Opera still renders the change properly. Usually.
I'll have to test Firefox 4's animation abilities...