With all due respect to people who insist on using Internet Exploder, I design sites in pixel font size instead of em’s or other scalable units because I want to know the site isn’t going to blow up with ugly word wrapping or content flowing beyond the containing element when a user agent resizes the text. But what does font size in pixels actually correspond to? Logic would suggest that it would be the distance from the top of the tallest ascender (say the top of the letter t) to the base of the lowest descender (say the tail of a y or g). But these seem to fall a bit short of the font size specified in CSS. Is there some arbitrary line just above and below those points which measurements include?