To quote the CSS 2.0 spec: The 'em' unit is equal to the computed value of the 'font-size'... Therefore, em is in units of font size, and is not independent o

To quote the CSS 2.0 spec: The 'em' unit is equal to the computed value of the 'font-size'... Therefore, em is in units of font size, and is not independent of it. It's totally unrelated to 'px' values, except by luck. Not all browsers default to 16px text. (And yes, it's true that px are actually defined as the distance subtended by an angle of 0.0227 degrees at a typical viewing distance of the device, but they're typically used interchangably with screen pixels)