The point in specifying fonts that you might consider "generic" is visual consistency. Different fonts end up being very different on the screen, and, as a desi

The point in specifying fonts that you might consider "generic" is visual consistency. Different fonts end up being very different on the screen, and, as a designer, your goal is to make your site look good across browsers and operating systems. If you have no way of knowing what fonts they're using it is very difficult to do that. Plus, the number of people who actually change their web browser font defaults is vanishingly small. Also, absolute font size specification used to be bad practice because browsers couldn't zoom very well. Now that all modern browsers (including IE7) have good full-page zooming it doesn't matter, and using exact pixels allow you to avoid browser rounding inconsistencies.