I did not follow the steps in Enable GPU support but hardware acceleration (f.e. sites with WebGL content) work fine. I ran nixos-generate-config which added something about intel in hardware-configuration.nix, and I enabled gnome desktop. I assume this handled the GPU acceleration?
Should I remove that section?
That section is terse, but it suggest to enable VA-API, which is for acceleration of video playback [1], not WebGL.
I think the section is incomplete though. I have VA-API enabled and chrome://gpu tells me:
Accelerated video decode is unavailable on Linux: 137247
Ah ok, for video, not WebGL. Someone will update that eventually then.