Unfortunately issues arise when using em's in Android webview compared to Chrome. Em's get rendered about 8x in webview, even though the Chrome emulator shows otherwise. I tried the % way but that doesn't fix it either.
In font sizes yes, other sizes not so much :-) (I couldn't reproduce the issue highlighted in musage's source by altering Firefox' default font size, but perhaps that's an IE only thing.)
In this case yes. But you can use em as a unit anywhere. Set your div elements to 20em width if you like.