You also need to be aware of the conditional commenting system of Internet Explorer often used with script includes or CSS. For example, a conditional comment-aware browser will read the following statement and will then include the patch.js file if it is an older Explorer version but not a newer version:
<!--[if lt IE 7]><script src="patch.js" type="text/javascript"></script><![endif]-->Browsers that do not understand such a comment extension will, of course, safely ignore the statement.