This is because the web designers in question think that specifying font sizes in pixels is the way to go, when in fact it is just about the worst possible thing to do. I've all but stopped reading these forums because of this --- even with huge fonts, the text is barely readable. Why can't people just leave the damn fontsize alone? Is it so hard?
font-size is for people who know what they are doing. If you're using font-size: xxpx, without trying to align something to an image (which is a daft thing to do in any case) you're not qualified to use that attribute.
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