[css-contain-2] Make content-visibility not animatable w3c#5708 (w3c#5709)

Author: vmpstr

css-contain-2/Overview.bs

@@ -1008,6 +1008,7 @@ Suppressing An Element's Contents Entirely: the 'content-visibility' property {#
 	Initial: visible
 	Inherited: no
 	Applies to: elements for which [=layout containment=] can apply
+	Animation type: not animatable
 	</pre>
 
 	The 'content-visibility' property