Intent to ship: CSS 'content: none' on (non-pseudo) elements

[Mats Palmgren]

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."
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1699964
Standard: https://drafts.csswg.org/css-content-3/#valdef-content-none
Platform coverage: all
Preference: layout.css.element-content-none.enabled
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
no effect.
Web Platform Tests: many included in the patch


/Mats