Internet Explorer 9 (and Chrome sometimes as well) tries to be a little too smart and do too many things for you. The best example is removing the Flash movie from the DOM when you set its style to:
display: none !important;.
This small gotcha led to some headaches about why JS wasn’t able to call methods (via the ExternalInterface) on my Flash movie. As it turned out this way due to IE9 just completely removing the Flash movie from the DOM.