A Comparison of Popular Online Fonts: Which is Best and When?
http://www.surl.org/usabilitynews/32/font.asp

Bernard, Michael, Melissa Mills, Michelle Peterson and Kelsey Storrer
Usability News
2001

Abstract:

A general survey of the Web finds that a majority of sites use 12-point fonts (size= 3) for much, if not all of their written content. With this in mind, we examined the most popular font types at this size for differences in effective reading speed (accuracy/reading time), as well the perception of font legibility.