[scroll-animations-1] ViewTimeline needs a subject arg, not a source arg. #7157

Author: fantasai

scroll-animations-1/rewrite.bs

@@ -346,7 +346,7 @@ spec: cssom-view-1; type: dfn;
 
 	<pre class="idl">
 		dictionary ViewTimelineOptions {
-		  Element source;
+		  Element subject;
 		  ScrollDirection axis = "block";
 		};
 
@@ -398,6 +398,10 @@ spec: cssom-view-1; type: dfn;
 
 			1.  Set the {{ViewTimeline/subject}} and {{ScrollTimeline/axis}} properties of |timeline|
 				to the corresponding values from |options|.
+
+			1. Set the {{ScrollTimeline/source}} of |timeline|
+				to the {{ViewTimeline/subject}}’s
+				nearest ancestor [=scroll container=] element.
 	</dl>
 
 	Inherited attributes:
@@ -422,8 +426,7 @@ spec: cssom-view-1; type: dfn;
 	</dl>
 
 	If the {{ScrollTimeline/source}} or {{ViewTimeline/subject}} of a {{ViewTimeline}}
-	is an element whose [=principal box=] does not exist
-	or is not a [=scroll container=],
+	is an element whose [=principal box=] does not exist,
 	then its {{AnimationTimeline/phase}} is the [=timeline inactive phase=].
 	The timeline is otherwise
 	* in the [=timeline before phase|before=] phase when {{AnimationTimeline/currentTime}} < {{ViewTimeline/startTime}}