This sample page shows an issue with Microsoft Edge: the :hover state stays applied even when the mouse is no longer on the button (tested with Microsoft Edge 40.15063.674.0, Microsoft EdgeHTML 15.15063).
To reproduce the issue: move the mouse from the button to the top. The tooltip (which was displayed in the mouseenter event of the button) is removed from the DOM in the mouseleave event of the button. The button stays blue whereas the :hover state should no longer be applied.