As far as I'm aware, any SPIRV isn't directly fed into the Vulkan backend, it's more like "WebGPU SPIRV => IR => validation => Vulkan SPIRV".

Also I wouldn't rule out that some browser vendors will accept SPIRV as WebGPU shader input as a non-standard extension one day (disclaimer: talking out of my ass here since I'm not on any of the WebGPU implementation teams). The WebGPU API is prepared for accepting different types of shader inputs (that's how the native implementations accept SPIRV instead of WGSL). I bet that this would solve exactly zero problems though ;)

> DirectX accepts SPIR-V nowadays

That doesn't mean much since SPIRV has different incompatible flavours, e.g. you can't feed a GL SPIRV blob into Vulkan, or a Vulkan SPIRV blob into D3D (does D3D actually already accept SPIRV or is this still in the 'planning stage'?)