ff2 mac: doesn't understand floating-point percentages (ie: 107.3% = 107%) - ran into this with font sizes.
ff3 thinks that if your parent is visibility:hidden, but you are visibility:visible, that you should display. (wrong.)
IE8 VHD from microsoft on vpc2007 is not the same as ie8 in the wild. Button element widths seem to be at least 1px wider in the wild.