The author has less control of the font than in a printed page. The author has less control of the formatting than in the printed page. The resolution of the medium is less. Fine detailed fonts do not therefore render well. Readers tend to scan rather than read.
Scroll bars add a new factor.
The browser's default font. The browser's default font size. The resolution of the monitor. In Windows, the display font settings. Best Practices Sans serif fonts are preferable to serif fonts.