I\'ve read a lot about the pros and cons of sizing with either relative or absolute font sizes. Fixed sizes don\'t zoom in IE6 but that\'s not much of an issue these days. A
think about your parents or grandparents looking at your web page, with and without their bifocals (or trifocals). If they can't read it because you made all the fonts 8px, they'll probably go somewhere else. If they click 'text size >> largest' and nothing happens, they'll probably go somewhere else.
given that - at least in the USA - a lot of web surfers are older people with time, money, and failing eyesight, the use of em for fonts for public web sites would seem to be of great benefit ;-)