From Information Week:
Users looking to file claims online for government help [from FEMA] must be running Microsoft Internet Explorer 6.0 or later with JavaScript enabled. That blocks everyone running Linux, Apple Macintosh computers, and Windows users running alternate browsers such as Firefox or Opera.
...Two weeks ago, the U.S. Copyright Office came under fire for a similar limitation when it said that its soon-to-be-launched revamped site will allow only Internet Explorer and a version of Netscape Navigator to access a copyright preregistration system.
This is really pretty simple. Make your web page standards compliant. Then all standards compliant browsers will be able to access your site.
If you think you need some proprietary Microsoft extensions then you don't know what you're doing.