Now for some specifics that affect your design.
As you probably know, font sizes on Macs and PCs are different. With the new versions of Explorer and Netscape, the same font spec on a Web page will make the type appear approximately the same size on both platforms in both browsers. If you ignore the concept of `point sizes' - which is different on Macs and PCs and only confuses the matter - it is good that they are now standardised. In the short term though, it is going to cause even more of a headache because you can no longer be certain that type on a Mac is going to be one size and on a PC, another - not that you ever could! A great many Mac- oriented sites are now going to display larger fonts than originally intended, making them look crude and `schoolbookish'.
Now it gets very messy. You have to check for the platform AND the browser AND the version. If you serve up alternative font specs for Mac and PC versions of Explorer and Netscape, now you also have to identify if they are version 4 (or less) or more than 4. Then, there is a further complication. You can't tell if the Mac Explorer 5 user has changed their default font size back to the original 12 point at 72 dpi specification or accepted the new 16 point at 96 dpi one.
A very thorough browser detection script from Netscape's developer site can tell you exactly what you are dealing with but is probably overkill for design purposes.