The D3D11 functional spec says 0 < w

I didn't see a constraint in a quick scan of the Vulkan spec.

For w = 0, primitive clipping would only allow points with (x,y,z) = (0,0,0). Everything else would be clipped. Are trivial points like

Is w < 0 valid? The equations in primitive clipping would say everything is discarded by clipping in this case. Is that required behaviour?

I think this needs clarification.