Chrome and Firefox felt there were sufficient problems with OpenGL driver availabity that they implemented WebGL using DirectX (via the ANGLE project), despite the more obvious mapping to OpenGL.

Is there any reason to believe that, unlike OpenGL, Vulkan drivers will be sufficiently widespread for browser vendors to use it on Windows?

That's really fascinating and I didn't know that.

Intel has no stated intention of releasing production-quality Vulkan drivers for Windows. Forum statement from August:

The current Plan Of Record is that Intel® is not supporting Vulkan on Windows drivers. The drivers that were made available on Developer.com are intended for Vulkan developers.

So, it is expected that some Vulkan drivers may not work for end users.

A quick google finds an Intel representative providing clarification: https://software.intel.com/en-us/forums/developing-games-and...

So, they ARE planning full Vulkan support on Intel iGPUs.

Good find, it's a shame that the only info is buried in forums and there's no timeline or official statements.