Apparently Internet Explorer 7 and earlier just don't care much for the Content-Type: header that servers reply, but they instead scan the body and guess what type it is. Thus "knowing better" than the content provider what content it truly is.
So in IE8 they're (according to that blog entry) introducing a new attribute to the Content-Type header. If the site also sets "authoritative=true" it means it really means the type and the browser will then actually believe the site. I can't stop giggling.
And yeah, some of the other craziness on that page is also good reading and they truly make you wonder what they are smoking during their brain storm meetings.