ios testing is getting worst by the year. new screen sizes are popping up all the time and from what I hear this year we will have a couple of new ones!
iOS is WebGL enable since 5, they just don't let you use it until it meets "some" standards, but i bet is on a close future
Develop games in your browser. Powerful, performant & highly capable.
I anticipate eventually most mobiles will support WebGL. (Apart from Windows 8...)
IE 11 comes as part of Windows 8.1 and should also be part of Windows Phone 8.1 and supports WebGL.