[css-display-4] Make 'display' animatable. w3c#6429

Author: fantasai

css-display-4/Overview.bs

@@ -225,7 +225,7 @@ Box Layout Modes: the 'display' property</h2>
 	Applies to: all elements
 	Inherited: no
 	Computed value: a pair of keywords representing the [=inner display type|inner=] and [=outer display type|outer=] display types plus optional ''list-item'' flag, or a <<display-internal>> or <<display-box>> keyword; see prose in a variety of specs for computation rules
-	Animation type:	not animatable
+	Animation type:	see [[#display-animation]]
 	</pre>
 
 	<p class=all-media>User agents are expected to support this property on all media, including non-visual ones.
@@ -857,6 +857,23 @@ The Root Element’s Principal Box</h3>
 
 	Additionally, a 'display' of ''display/contents'' computes to ''block'' on the root element.
 
+<h3 id='display-animation'>
+Animating and Interpolating 'display'</h3>
+
+	In general,
+	the 'display' property’s [=animation type=] is [=discrete=].
+	However--
+	similar to when animating 'visibility'
+	(see [[web-animations-1#animating-visibility]])--
+	during interpolation between ''display/none''
+	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.
+
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@@ -2091,6 +2108,7 @@ Additions since Level 3</h3>
 
 	* the 'layout-order' and 'reading-order' [=longhand properties=] of 'order'
 		(<a href="https://github.com/w3c/csswg-drafts/issues/7387#issuecomment-1217193918">Issue 7387</a>)
+	* the ability to animate 'display'
 
 <h2 id="priv" class="no-num">
 Privacy Considerations</h2>