Is anisotropic filtering standardized by the D3D or OpenGL specs now? I know in the last decade or so, it was still the case that if you did an image diff of the same scene being rendered with say, 8xAF on both NVIDIA and AMD GPUs, the result would be different since both vendors used a mix of heuristics to get good results. I'm guessing they might've dropped the hacks by now and converged on a known 'good' algorithm. I wouldn't be surprised if mobile GPUs are still cheating even if desktop ones have stopped, though, so you'd be able to detect that with WebGL.

There's also the 'detect driver bugs' approach since there are test suites out there that detect various driver stack bugs using special shaders, you could run a set of those to identify the GPU (or even driver) being used to rasterize the WebGL content.

> fairly well

Don't we still live in an age where GPU drivers need game-specific driver fixes in order to work correctly? Being bug free is a different question than being free from any noticeable difference in the output, and we seem to not even have reached the bug freedom stage yet.

I mean even Firefox Webrender is only available to Nvidia GPUs at the start, no?

The driver fixes are usually performance improvements where the vendor rewrites shaders to make the game run better or add support for weird vendor features (like SLI). If the vendor had to hack around the game to make it work at all, the developers wouldn't have been able to even test the game during development.

WebRender was locked to NVIDIA as a known target with a known set of driver bugs. They could've chosen AMD or Intel as the target instead and worked to figure out all the relevant driver bugs and worked around them, but for whatever reason they picked NV.