Running an application in IE9 does not necessarily mean you are running IE9 standards mode. IE9 has many modes that can be defined in the page head tag (see the “Document Mode” section below). You can overwrite the page mode manually by selecting F12, where you can set both browser mode and document mode.
GWT IE permutations work best with each version of “standards” mode. Mixing modes, say browser mode = 7 and document mode = 9, is not recommended and the behavior is undefined. To keep it simple, try to keep browser modes and document modes the same. If you must use mixed mode, be aware that you may run into issues that are still not supported. The exception is if you are emulating an older browser when you still do not support the new version, for instance, you emulate IE7 (EmulateIE7) on IE9.
http://www.gwtproject.org/doc/latest/DevGuideIE9.html