Chris Messina links to this somewhat handy Em Calculator. It claims to make it easier to do pixel-perfect design without compromising accessibility. My understanding was that this is kind of innacurate. most browsers don’t render more than two decimal places worth of Em. Since I prefer to set the default at 10px (it makes the math easy), that means setting my em’s to (0.625) which most browser will truncate to 0.63. I’ve always (since reading it in Matthew Stephens‘ hilariously commented CSS) used the 76% method in my body, from sane CSS typography, and then used em’s to style the remaining elements.