What? If it isn't in the browser, what's the difference between that and something you download on Steam?
The final dream is having a unified, Desktop, Consoles (Ps4 and Switch) and Browser runtime with a unified API (where sensible). One can dream.
With the browser, you're just adding another abstraction layer that has even more complexity. WebGL is far for a decent spec. And we're talking just graphics, as I mentioned before, audio and networking have tons of unstable and untested code.
Multimedia programming is already hard on native environments. I don't see the web being a friendly environment anytime soon. There's no silver bullet, "code once, run anywhere" for games and multimedia.