When things get slow on web design mailing lists, someone will inevitably bring up the difference in font sizes between Mac OS and Windows, and a deluge of solutions will follow. It's no longer an issue. Before you scream bloody murder, Netscape is rumored to be planning on using the same setting in Gecko. Here's the rationale. The web is authored by everyone from advanced designers and programmers to absolute beginners.
Beginners tend to design according to what works and looks good on their system.
On the Windows platform, with its larger fonts, 8pt type looks good. Thus, 8pts is the same as 8 pixels. And 8 pixels is at least one pixel too few to render type legibly. Since CSS came into use, Mac users have been hitting pages they could not read, because Windows-based web designers had specified 8pt type in the style sheet. By switching to a 96ppi default, IE5 solves this long-standing problem.