Your usage logs can usually tell you the browsers, platforms, and versions used by your visitors. Generally speaking, your site probably needs to work fine from at least versions 4 and up of the two major browsers, Netscape and Internet Explorer, for both Macs and PCs. Netscape 6 recently came out, and there may be some surprises with how your site works. So it's important to check it out and make sure your site renders OK in it, too. AOL's browser, while using Internet Explorer, nonetheless has some customization, so it's good to make sure your site works well in that, too.
Accessibility needs to always be considered, too, of course.
It's important to allow for these possibilities. Government-related sites require it, for example. Each site has different statistics, so the above relevance depends on your particular site. Your ISP typically has information about how to do this. If they don't happen to offer this, there are free stats logs available, too, such as the Counter .