The reason it's showing that is because the document says

    <?xml-stylesheet type="text/xsl" href="rfc2629.xslt" ?>

But rfc2629.xslt isn't uploaded in the same directory (and Firefox gets real unhappy when it tries to parse the HTML 404 page as an XSLT file). Which is fine, the XML file isn't there to show it rendered, it's to download the source. So you can still just save it.