This was an inline issue in the spec, and seems trivial enough to resolve that I'm writing it up as a hopeful tacit resolution candiate. Currently the GPUPrimitiveState has a cullMode that defaults to "none", but the other possible cull modes ("front" and "back") don't apply if the topology is a point or line type.

In some other cases, such as stripIndexFormat, we've required that the value be left undefined when it doesn't apply, so we could do the same here.

On the other hand, there's no particular issue with developers specifying a cull mode in these cases. It's not validated on any of the native APIs, doesn't cause adverse effects when rendering, and the default is such that developers who specify it are really just making more work for themselves.

My preference is to simply not validate it. There's no technical reason to block developers from setting this, and doing so mildly breaks backwards compatibility.