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While the spec currently requires a string value that converts to a number between 0 and 1, only IE conforms to this. Gecko, WebKit and Blink expects a percentage string e.g. "50%"
Fixed per WG resolution of 2014-04-02 [1] - RESOLVED: Use percentage values for key arguments and map from/to keywords to 0% and 100% respectively. [1] http://lists.w3.org/Archives/Public/www-style/2014Apr/0016.html