[scroll-animations-1] Convert view-timeline-inset to take start/end pairs per timeline. w3c#7695

Author: fantasai

scroll-animations-1/Overview.bs

@@ -486,19 +486,23 @@ spec: cssom-view-1; type: dfn;
 
 	<pre class='propdef'>
 	Name: view-timeline-inset
-	Value: [ auto | <<length-percentage>> ]{1,4}
+	Value: [ [ auto | <<length-percentage>> ]{1,2} ]#
 	Initial: 0
 	Applies to: all elements
 	Inherited: no
 	Percentages: relative to the corresponding dimension of the relevant scrollport
-	Computed value: per side, the keyword ''view-timeline-inset/auto'' or a computed <<length-percentage>> value
+	Computed value: a list consisting of two-value pairs representing the start and end insets each as either the keyword ''view-timeline-inset/auto'' or a computed <<length-percentage>> value
 	Animation type: by computed value type
 	</pre>
 
 	Specifies an inset (positive) or outset (negative) adjustment of the [=scrollport=]
 	when determining whether the box is in view
-	when setting the bounds of a [=view progress timeline=].
-	The resulting rectangle is the <dfn>view progress rectangle</dfn>.
+	when setting the bounds of the corresponding [=view progress timeline=].
+	The first value represents the [=start=] inset in the relevant axis;
+	the second value represents the [=end=] inset.
+	If the second value is omitted, it is set to the first.
+	The resulting range of the [=scrollport=] is
+	the <dfn>view progress visibility range</dfn>.
 
 	<dl dfn-for=view-timeline-inset dfn-type=value>
 		<dt><dfn>auto</dfn>
@@ -511,12 +515,10 @@ spec: cssom-view-1; type: dfn;
 			defines an inward offset from the corresponding edge of the scrollport.
 	</dl>
 
-	The 'view-timeline-inset' property assigns its values to each side
-	exactly as the 'margin' property does.
-
-	ISSUE: Should the initial value be zero or auto?
-
-	ISSUE: Do we need all the longhands? Seems like overkill...
+	If 'view-timeline-name' has more names than 'view-timeline-inset' has specified inset pairs,
+	the excess timelines use the last 'view-timeline-inset' pair.
+	If 'view-timeline-name' has fewer names than 'view-timeline-inset' has specified inset pairs
+	the used 'view-timeline-inset' list is truncated.
 
 ### View Timeline Shorthand: the 'view-timeline' shorthand ### {#view-timeline-shorthand}