[css-contain-3] Make content-visibility animatable. (#8737)

Allow animating content-visibility as animation type discrete with
special interpolation to avoid hidden values similar to visibility.
Fixes #8627.

css-contain-3/Overview.bs

@@ -1066,8 +1066,30 @@ The <code>CSSContainerRule</code> interface</h3>
 Suppressing An Element’s Contents Entirely: the 'content-visibility' property {#content-visibility}
 =================
 
+	<pre class=propdef>
+	Name: content-visibility
+	Value: visible | auto | hidden
+	Initial: visible
+	Inherited: no
+	Applies to: elements for which [=size containment=] can apply
+	Animation type: see [[#content-visibility-animation]]
+	</pre>
+
 	Issue: [[css-contain-2#content-visibility]]
 
+<h3 id='content-visibility-animation'>
+Animating and Interpolating 'content-visibility'</h3>
+
+	In general,
+	the 'content-visibility' property’s [=animation type=] is [=discrete=].
+	However,
+	similar to interpolation of 'visibility'
+	(see [[web-animations-1#animating-visibility]]),
+	during interpolation between ''content-visibility/hidden''
+	and any other 'content-visibility' value,
+	<var>p</var> values between 0 and 1
+	map to the non-''content-visibility/hidden'' value.
+
 Privacy Considerations {#privacy}
 =================================
 
@@ -1103,6 +1125,8 @@ Changes since the 18 August 2022 Working Draft</h3>
 	* Correct typo in 'container-type' syntax, to clarify that ''normal'' cannot
 		be combined with other values.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/7669">Issue 7669</a>)
+	* Make the 'content-visibility' property animatable.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/8627">Issue 8627</a>)
 
 <h3 id="changes-2021-12">
 Changes since the 21 December 2021 First Public Working Draft</h3>