The key is to use features so that they degrade gracefully in non-supporting browsers, and to avoid dependence on poorly supported features.
Style sheets are an excellent example of a feature that degrades gracefully.
The proper use of style sheets can significantly enhance the presentation of a site while not deterring from the site's accessibility at all in non-supporting browsers. On the Web, there is no lowest common denominator, since new features can often be used safely, even on non-supporting browsers. Video, sound, images, and Java applets can all have alternative content provided to those who are unable or unwilling to experience these features. Authors can show off the Web's multimedia nature without sacrificing the accessibility of their pages. Netscape is currently the most popular browser on the Web, but there are a significant number of non-Netscape users.