[css-display-4] 'display: none' in keyframes becomes 'revert-layer'. Address inertness. w3c#7778

Author: fantasai

css-display-4/Overview.bs

@@ -869,10 +869,15 @@ Animating and Interpolating 'display'</h3>
 	and any other 'display' value,
 	<var>p</var> values between 0 and 1
 	map to the non-''display/none'' value.
-
 	Additionally,
-	the 'display' property is ignored
-	when specified in <a spec=web-animations-1>animation effects</a> or ''@keyframes'' rules.
+	the element is [=inert=]
+	as long as its 'display' value would compute to ''display/none''
+	when ignoring the Transitions and Animations [=cascade origins=].
+
+	Because a value of ''display/none'' would interrupt the animation,
+	a declared value of ''display/none''
+	in an <a spec=web-animations-1>animation effect</a> or ''@keyframes'' rule
+	is replaced with ''display/revert-layer''.
 
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