To play devil's advocate: Isn't a big focus of web tech the goal of supporting multiple backends for everything? If a new platform wants to support WebGPU but not Vulkan (ex: XBone or PS4) it should be not just theoretically possible, but actively supported by the design of the Web spec.
But, that's assuming that it's not possible to write a wrapper on top of DX (for Xbone) that proves a Vulkan API. It's already been shown to be possible to do something similar, with MoltenVK.
The ball would be in Microsoft's court for this one.
> But, that's assuming that it's not possible to write a wrapper on top of DX (for Xbone) that proves a Vulkan API.
Is this not how Vulkan on Windows works? Given how many OpenGL drivers were just shims over DirectX in the past, I wouldnt be surprised if the same course was charted there.