Performance issues are probably the single biggest problem you will encounter when developing a browser-based mobile game, the main reason being that a wide variety of devices is available, each with very different capabilities.
Even if you chose to support only iOS, which is probably the simplest ecosystem at the moment, you will still have very large differences in performance, screen resolution, and browser support.
To get an idea of the complexity of the situation, take a look at the supported device for jQuery Mobile (http://jquerymobile.com/gbs/). For your game, you should probably have an approach similar to theirs; select a few device/software versions you will target. Your game should work flawlessly on those.
Then make sure that the game runs without errors on a broader selection of devices. On those devices, performance can be less than ideal. Finally, draw a clear line beyond which you won't even bother to test whether your game runs at...