With the ability to change default font sizes in a browser and to change the gamma with GammaToggleFKEY, it is fairly easy to simulate a PC browsing environment on a Mac screen, but wait, the differences between Mac and PC versions of Netscape and MsIE are even more of a headache and you can't so easily simulate those. You either need to have a real PC or a Windows emulator like SoftWindows or VirtualPC.
The differences between the 3 and 4 versions of Netscape and MsIE provide few surprises on the Mac platform. It is easy to imagine that the Windows versions are equally similar - no way! You need to check your pages in version 3 and 4 of both browsers on both platforms and adjust them accordingly to get the best overall result. Be warned though, getting versions 3 and 4 of Internet Explorer to co-exist on the same PC is no easy task. The trick is to use the 16-bit (Windows 3.x) version of MsIE 3 and the 32-bit (Win95/98/NT) version of MsIE 4.x.