There's precedent: Chrome and Firefox both implement WebGL on Windows with ANGLE, which translates OpenGL to Direct3D.

https://en.wikipedia.org/wiki/ANGLE_(software)

And it seems they are working on a more generic library : https://github.com/google/nxt-standalone

Ah, and NXT has a Metal backend! Very cool.

It's important to note that NXT isn't necessarily a replacement for ANGLE. It's an experimental replacement for WebGL as a whole, with a different API. There still needs to be a way to run WebGL programs on Mac if this deprecation leads to removal a few versions down the line.

And not strictly a replacement either, WebGL would not go away. The successor to WebGL is still so far away that there will probably be some versions after WebGL 2.0.

browsers are moving to webgpu

https://lists.w3.org/Archives/Public/public-gpu/