The argument on performance comes from the idea that theoretically the rules are applied in order. So the browser has built in stylesheet of 8px, and after that

The argument on performance comes from the idea that theoretically the rules are applied in order. So the browser has built in stylesheet of 8px, and after that a margin of 0 is calculated. (And after that, some more specific margin in the stylesheet is recalculated) Basically * { margin: anything } adds one extra calculation to each element in the page. Not sure if the performace hit is measurable tough, knowing how much optimization goes into browser engines.