Unless you have a good reason, I urge you to never do that. Even ignoring accessibility (which is pretty much completely ruined in this situation no matter what), you'll need to reinvent every single primitive widget you need yourself and have it work the same as it does on all platforms.

That's a shitload of code shipped to the user before you even write a single byte of code for your application!

If you write all the rendering yourself, you can just output to a canvas but that kind of defeats the purpose. Having a JS layer for WebGL that calls into WEBASM for calculations etc should work fine though.