Summary: spec quote: "On elements, this inhibits the children of
the element from being rendered as children of this element, as if the
element was empty."
Platform coverage: all
Other browsers: as far as we know, no other UA support this feature yet,
so there's some web-compat risk in shipping this in case authors are
unknowingly setting 'content: none' on elements and depend on it to have
Web Platform Tests: many included in the patch