When designing a recent game, I used 1280 x 720 resolution window. It worked fine on Android but on iPad i see white bars on top and bottom of screen. I was using Letterbox scale.
To solve the problem, I followed the advice here.
And it worked. Bascially, you need to set your window to 16 : 9 aspect ratio, eg, 1280 x 720, or 1600 x 900 etc. and use Scale Inner.
So, I used 1280 x 720 and Scale Inner for the property "Fullscreen in browser" and some of the sprites on the left and right edges got cut off. So, I moved them inwards on both sides, and it worked beautifully.
I chose 1280 x 720 instead of 1600 x 900 because, if you chose the latter, your background images might take up a lot of memory, making your game larger, especially if you are doing a perpetual-side-scrolling game which uses multiple background layers for parallax effects.