[css-contain-2] top-layer makes an element relevant to the user. w3c#6727

Author: tabatkins

css-contain-2/Overview.bs

@@ -1374,6 +1374,9 @@ Suppressing An Element's Contents Entirely: the 'content-visibility' property {#
 		* Either the element or its [=contents=]
 			are selected,
 			as described in [[css-pseudo-4#highlight-pseudos]].
+
+		* Either the element or its [=contents=]
+			are placed in the [=top layer=].
 	</div>