Please ignore non-existing pseudo-class on pseudo-element vendor extension

[mthuurne]

Validate the following fragment while treating vendor extensions as warnings:

::-webkit-scrollbar:vertical {
	border: 1px solid black;
}

The validator will report the ::-webkit-scrollbar pseudo-element vendor extension as a warning, as expected. However, it reports the :vertical pseudo-class as an error. Maybe that is formally correct, but from a practical point of view, I would very much prefer if vertical would be ignored because it applies to a vendor extension.

[ylafon]

To me it is more a bug than en enhancement as the initial goal was what you described.