I was amazed that he had found a cross-browser way to make sure font-sizes match pixels. But when testing it i found that his 12 pixel header is 8 pixels on my

I was amazed that he had found a cross-browser way to make sure font-sizes match pixels. But when testing it i found that his 12 pixel header is 8 pixels on my screen (smaller) and his 36 pixel heading is 40 pixels (larger) :-( that's with a plain firefox. Am I the only one experiencing this, or is this feature just not usefull?