This site explains it quite well:
Mark's text has a great flow making it astonishinly easy to read.
You might also want to check the "further reading" links.
> The bane of my life at the present is IEWin's ridiculous tendency to
> render 'keyword-sized' (i.e. x-small, small etc) text one size larger
> than every other browser/platform combination in the known universe. I
> like using keywords, it helps me avoid the tendency to try to control
> text size down to the last pixel. But clients who inevitably use IEWin
> want the text size they see to be smaller and I don't like leaving all
> other users squinting by default.
> So, can anyone suggest anything to help a web developer trying really
> hard to do the right thing? ;)