But as far as what APIs you can expose to JS there is a limit. Can you expose WebGL2? Fullscreen? Screen Orientatio API? WebVR? WebAssembly? Input type=Color, OffscreenCanvas? or do you have to wait for Apple to add them to the base iOS WebKit? Can you fix the fact that iOS Safari shows very poor performance animating while streaming assets?
There is much more to the browser than rendering engine.