This is a tough question; there is definitely some content such as sounds and images which will never be universally available to blind or deaf users. However for the most part, staying standards compliant and making sure that your pages have no syntax errors or dependency on a specific browser is the best way to be sure of reaching the widest possible audience.

The canonical reference for the most popular and most widely supported markup languages, HTML and XHTML, is published by the World Wide Web Consortium. This documentation is the agreed-upon standard for web markup across the entire internet. It is likely to be supported by all browsers, from cell phones and text-only browsers for the blind to the latest version of Internet Explorer.

To be sure you've created pages that not only meet the standards but fail to have any content-impairing typos, you should probably run them through the XHTML and HTML validator and potentially through the CSS validator as well, if you have used cascading style sheets for further page layout. Be warned that older or less capable browsers may not support CSS so do try reading your page without the CSS in the source to see if all text displays fully and legibly in its absence.

Although it is popular to write browser-specific code targetting IE or mozilla/netscape, and although such code may work on the majority of the browsers available today, you do limit your site's usefulness by doing so. Consider whether you think the cute tricks with the cursor or ActiveX controls are worth someone not being able to find the link to your resume or research. Old, small, and handicapped accessible browsers may be unable to use your site and often will just ignore it instead of informing you. The "Viewable With Any Browser" campaign may serve to further enlighten you as to the rationale behind creating more compatible web pages. Do consider making your page fully handicapped accessible.