Disclaimer: I work on the team at Google responsible for the open source translator engine that runs under WebGL (ANGLE)

WebGL on Linux requires the OpenGL backend until eventually Vulkan is supported well enough.

Apple's WebGL2-on-Metal approach involves the same translator engine I work on, and was very much a collaborative effort between Google, Apple, and external contributors. Chrome will adopt this in the future after integration work is finished.

I can confirm that browser GPU programmers are definitely spread pretty thin just ensuring compatibility and reliability across all platforms