Website designers, in general, target the mainstream. Often that means only testing websites on the latest version of Internet Explorer running on Windows, which is a mistake. It's worth trying to make web pages conform to web standards (and then test to see if the major web browsers support your use of standards well enough), since creating pages to work with particular browsers is always a chase of a moving target.
It's true that the vast majority of people coming to this site use either Windows or a Mac, and either Internet Explorer, Safari, or one of the browsers based on Mozilla/Netscape. Looking at my site statistics, those combinations make up about 92.3% of visits here in 2003 so far. Another 1% are Linux users, and 5.1% are "unknown"—my web server couldn't find out what kinds of systems they were, but it's a good bet that the proportions are the same as among "known" types (there might be some Palm and smartphone users too). But what about the remaining 1.6%? Here's how it breaks down:
|Operating System||Hits 2003|
|Unknown Unix system||241|
Those are small but non-trivial numbers. Most of them are Unix or Unix-derived systems of some sort, from Sun, Hewlett-Packard, IBM, SGI, or one of the various open-source projects out there. But look: there are users of the Amiga, IBM's old OS/2, and even CP/M, which was on its way out when I started using computers more than 20 years ago. Most surprising, someone has come here using a Sega Dreamcast video game console.
The lesson here is that you can't possibly know what kinds of weird devices might visit your website. I wouldn't even want to try finding a Dreamcast or CP/M machine to try testing this site on. So I don't. I just try to make the site valid HTML, and hope people can read it okay when they get here.