CSS: converting px to em

I got an email from a friend today asking me if i knew anything about converting px to em to make a site truly accessable. I didnt know the answer but got an email back from him a bit later pointing me to here which has some good discussion on the subject. Not only that but a couple of good links to. One of them being here where the author has gone to the trouble to take about 264 screen shots of different font-sizes on different browsers.