Hello,
I made a game for Android and iOS, Then port it to webgl and all seems ok when I play on a browser.
But on mobile browser quality is downscale (looks blurry). Then this prevent performance drop but how to achieve a better graphics quality on mobile browser ?
Make a native app. 
Or wait, you have that already?
Why are you porting it to WebGL and then backport to mobile?
In any case, you cannot expect nearly the same quality as native apps in the browser.
I have already make it native. I port to webgl but some players maybe will access it from their mobile.
Then I try to have the best experience in any case.