Serving a basic stylesheet to old browsers
CSS is now used so extensively on the web that users of really old browsers such as Netscape 4 are going to have a pretty poor experience all in all. However, we can still be kind to users of really old browsers by at least making sure our advanced use of CSS doesn't crash their browser or cause the content to be completely unreadable. The way we do this is to serve a very simple stylesheet to these browsers and attach our real stylesheet using a manner that they do not understand.