I’m not sure if it’s possible. If is, I think that running in the browser could cause some performance issues. Also I’m not sure how such game would deal with touch interface (it would be running on Internet Explorer which interprets touch gestures on its own way). As a Graphic/UX designer I always think about this. Native UE4 support will give more freedom and stability. I’m Windows Phone user and I really hope that Epic guys plan to implement this soon.
Michal