making some tests and I think simple games can be done with HTML canvas for more complex we can use WebGL
500 bodies and still 60 FPS. i can tell you that having 500 bodies in a single window where all are drawn is not simple! most games do not have so many bodies at the same time for a long period. mostly they are killed some when or outside the frames and also we are talking about dynamic bodies. static bodies take much fewer calculations.
to improve performance when we will draw sprites on our canvas we can use 2,3 or even more canvas where we can on 1 draw all background sprites and on the second we draw only dynamic bodies,... so with logical thinking and creativity, we can create really nice games for the web
so I am thinking of 2 possibilities.
1, create a framework where people can write b4x code and it will be exported as JS
2, create a whole game creator app in b4j where people can create apps without using code. just drag&drop approach with the possibility of settings for bodies etc. like construct or game maker,...