There's an active attempt to make a DOM rendering engine in Rust using these APIs. https://github.com/DioxusLabs/blitz

That looks like its own thing, so not going to benefit Servo and Firefox if I understand correctly? Since they are using WebRender.

Correct. There are some shared components (notably Stylo, the style system), but Webrender is not one of them. Webrender is still maintained primarily by Mozilla as part of Firefox. If they have any plans to move to Vulkan/Metal/DX10/wgpu then I haven't heard of them.

It might be possible for Servo to go down the same route as Blitz and have pluggable rendering backends. If so then the wgpu-based renderering library we are using (Vello [0] - which is an exciting project in it's own right) could be an option. Servo is actively looking at potentially using this library to implement Canvas2D.

[0]: https://github.com/linebender/vello

Wouldn't be better for them to use wgpu instead?

Given that wgpu is based on WebGPU, it is kind of limited by a graphics API designed for the Web sandbox and managed languages.

Not sure, I don't see an issue with using Vulkan directly, but I think WebRender developers did plan to use wgpu at some point.