There are also some sites that actually require features of MSIE that don't exist in other browsers.
And some sites depend on bugs in MSIE, such as its ignoring of Content-type and inferring the type from some other input such as the initial page content. For instance, I ran into a traceroute page that was outputting HTML, but the MIME type reported was text/plain, so Moz displayed the source instead of the rendered HTML.
