.NET is partially compatible with older browsers. .NET does require a minimal level of JavaScript support, and it will attempt to adjust the HTML returned to the client based on the client's browser's capabilities. However, for browsers other than Microsoft Internet Explorer, .NET may adjust content by disabling or ignoring certain features. For example, client-side validation only works with Internet Explorer; other browsers only get server-side validation.
Moreover, the rendered pages may not look good because certain older browsers have inconsistent and incomplete support for standard HTML, CSS, and JavaScript. This is especially true with Netscape 4.0, which, for example, supports approximately 30% of the CSS1 specification. So it is possible that an application might function normally in terms of navigation, user input, etc. on an older browser, but it would probably not look good.
Internet Explorer Browser Compatibility