There are over 1 million sites with notices stating that a “browser is not supported“, typically in large, bold type at the top of the page. Why would a site choose not to support a browser? Browsers should only be expected to render sites which follow the standards provided by the W3C. Nothing more, nothing less. If you stick to the standards and ignore tags or features that are only implemented by individual browsers then your site will support all of the major browsers. It’s quite simple really.
I’ve said it before, and I’ll say it again. Web sites exist primarily to provide information. By using non-standard tags, you limit who can access your information thereby frustrating the original intent of the internet. Validate your HTML and all will be well.