It's all about "forcing" with MS stuff. *Force* IIS to serve a header which will
*force* IE not to use it's shitty compatibility mode, even if it's an intranet
site and the client has "display intranet sites in compatibility view" ticked in
their compatibility prefs.
The alternative is to include the equivalent <meta http-equiv...> in your
markup, but that will sometimes be ignored.
This header is discouraged by MS for production environments. Then again, MS is