Keep things available
Three more keywords to bear in mind:
Compatibility, Accessibility and Speed.
Make sure they all check out. A poor performance in any one will keep a slice of the market away from you. Visitors should be able to comfortably read your site and its content on whatever they happen to be using - whether it's a super fast office link or the shared slow pipe down at the library public access terminals, your pages should be viewable, and quickly.
If it doesn't work comfortably and attractively in both the Netscape and Microsoft browsers from Explorer 7 onwards at the very least, it needs a rework. Many people have no choice which plug-ins and add-ons they have, so the increasingly popular Macromedia Flash inserts won't necessarily hit everyone.
Remember that your visitors are your customers, and that you are there for them - don't imagine that you are in a position to dictate to them how and where they can view your content, or to make them install new software before they can visit your site, unless you really do want to pick and choose your customers, and turn some away.
If you try and exclude them in this way, the chances are that a high number will just say, "Forget it. I'll go elsewhere" rather than "Ok, I'll go get the new browser and then come back."
Making your Web site a success is a many sided task - some of it is work up front in design and development; some of it is ongoing, never-ending, now until retirement commitment.