One pitfall that people encounter when analyzing the information about their current visitors is to use that information to reinforce exclusions. Take for example the common phrase, "My visitors don't use insert browser, operating system or platform here so I don't have to design for it." This is simply faulty logic. In most of these cases, people are not using that browser, operating system or platform because the site either does not support it at all or supports it very poorly. They are not visiting because the site in question has failed to account for their needs. So how do we account for the needs of people we don't know?
Good news! People have already thought about making the web usable for everyone and come up with some solutions. Those solutions have been formalized into accessibility guidelines.
If you're not familiar with accessibility guidelines, you can get a detailed account of accessibility at the W3C. For our purposes, it's sufficient to say that meeting accessibility requirements will ensure that practically any web user will be able to view and use your web site. That may not sound particularly impressive, but if you've spent time on the web using a browser other than Internet Explorer, you can appreciate the fact that some web pages either don't work well or won't work at all with some browsers. Accessibility guidelines will help you avoid many of these problems.
Accessible web pages will also help you with an audience we haven't discussed yet. This audience segment is one that makes the web significantly different than traditional media. That segment is made up of web crawling robots. Though robots can be used for many different purposes, the robots webmasters are most often concerned with are search engine robots. What make this portion of our audience an important consideration is that they are often forgotten in discussions about audience. Robots are critical to a web site ranking in search engines as they are the first step in the process of getting your web pages ranked, yet they do not view the web through human eyes so they require special consideration.
Robots view the web very differently than most humans. To being with they will parse a page rather than render it as a browser would. There can be significant differences between the two. As a result, robots only look at the raw content on a page whereas most people see the web as a visually engaging medium. If you are using Linux or UNIX, give lynx a try to help get a rough approximation of what a robot sees when it surfs the web.
Whether this audience segment is important to your site is a decision you can make at a later time, but in either case, it's important to recognize and understand this significant demographic.