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@chriscoyier yeah I reckon just say no support for IE9 since it's a broken implementation anyway. Your proposed test will fail in IE9 so that should be fine.
I modified my testcase based of your idea to test for 50vw width as it would false positive in Firefox doing 100vw as the div it injected would naturally be the full window width too.
This is false in IE9 and true in Chrome 20 and IE10