I was struggling with the same issue for mobile. I was thinking of loading the background dynamically, and storing a ton of sizes and aspect rations on the server as images. If there is not an exact fit, I would either get a larger one and crop or scale down, or smaller and up.
However, what I found was that although my game only displayed a few sprites at a time, there were so many outside the game (as you need to have sprites for every thing which can be touched or moved). The core game was fast, but when I added all the dumb info screens and their backgrounds, buttons, settings and all the stuff you need to put round the game, it was too slow and the code unattainable so abandoned the project even though the games themselves were complete (it was a suite of slot machines for real money for an existing online casino).
However, I'm keeping my eye on construct 2's progress, hoping that one day it will have more support for native html 5 mobile, e.g. the ability to swipe in a settings form and a better solution for multiple screen sizes/ratios without black letterbox bars (which is currently possible, but extremely difficult when you add swipe navigation). When It does, the world we be at our feet.