I have this problem and maybe I\'m not the only..
If I test my website on FF, Chrome, Safari, IE7, IE8 is perfect.
On IE9 (standard view) It doesn\'t load CS
IE9 is known to reject stylesheets which are not sent using "text/css" MIME type. This is a new security enhancement, but it's catching a few people off guard. Is your CSS file dynamically generated? Make sure its getting passed as 'text/css'.
More info: http://blogs.msdn.com/b/ieinternals/archive/2011/03/27/http-406-not-acceptable-php-ie9-standards-mode-accepts-only-text_2f00_css-for-stylesheets.aspx
Another try:: It's not just 'text/css' in the doc that needs to be correct, you need to make sure your local IIS is sending it with the proper headers. "If a style sheet is ignored due to an incorrect MIME-type, your site may fail to render as expected. Text, images, or other features may lack the desired styling. If a style sheet is ignored because it does not bear the correct MIME-type, a notification will be logged in the IE9 F12 Developer Tools console."
http://msdn.microsoft.com/en-us/library/gg622939%28VS.85%29.aspx
IE9 Not applying linked style sheets