Responsible web developers test their systems in all popular web browsers.
I personally don’t bother, because, really, you don’t really expect me to run Windows, do you?
A recent A List Apart article describes using a low-end Mac as a cross-platform web design testing station. The article is insanely detailed, and it inspired me to restore my own ancient cross-platform testbed, which was a copy of VirtualPC 1.0 (dating from about 1997).
Because the ALA article recommended it, and because it is a lot cheaper, I bought a copy of FWB’s RealPC. This was a big mistake. The electronic version is incomplete and broken: the documentation refers to files that are not supplied, for example. So do FWB’s support reps. The site-search at fwb.com points to a CGI that does not exist as of this writing. And when I tried to call FWB, all their phones were misrouted. I returned the software for a refund.
Next I purchased and downloaded VirtualPC from Connectix. So far, I’m impressed; it’s well worth the extra $70. It took an evening to install Win98, IE5, IE5.5, and IE6, but now I’m about 20 seconds away from testing a web page in any of those browsers. Considering that Microsoft makes it impossible (AFAIK) to run multiple versions of IE on one computer (well, within a single installation of Windows), this emulated solution is significantly faster: I can save the state of these virtual machines, with IE already running, so I can jump from the MacOS into already-running IE within Windows, in mere seconds.
To be fair, Windows emulators are also available for Linux and even Windows; this is not strictly a Macintosh solution. The point is that for testing web systems, emulation is absolutely the way to go.