[scroll-animations-1] Switch ViewTimeline start/end to using lengths, now that currentTime is percentages. w3c#7312

Author: fantasai

scroll-animations-1/rewrite.bs

@@ -357,8 +357,8 @@ spec: cssom-view-1; type: dfn;
 		interface ViewTimeline : ScrollTimeline {
 		  constructor(optional ViewTimelineOptions options = {});
 		  readonly attribute Element subject;
-		  readonly attribute CSSNumberish startTime;
-		  readonly attribute CSSNumberish endTime;
+		  readonly attribute CSSNumericValue startOffset;
+		  readonly attribute CSSNumericValue endOffset;
 		};
 	</pre>
 
@@ -373,25 +373,25 @@ spec: cssom-view-1; type: dfn;
 		::  The element whose [=principal box=]’s visibility in the [=scrollport=]
 			defines the progress of the timeline.
 
-		:   <dfn>startTime</dfn>
+		:   <dfn>startOffset</dfn>
 		::  Represents the starting (0% progress) scroll position
 			of the [=view progress timeline=]
-			as a percentage {{CSSUnitValue}}
-			representing its [=scroll container=]’s scroll progress at that position.
+			as a length offset (in ''px'') from the scroll origin.
 			Null when the timeline is [=timeline inactive state|inactive=].
 
-		:   <dfn>endTime</dfn>
-		::  Represents the ending (100%) scroll position
+		:   <dfn>endOffset</dfn>
+		::  Represents the ending (100% progress) scroll position
 			of the [=view progress timeline=]
-			as a percentage {{CSSUnitValue}}
-			representing its [=scroll container=]’s scroll progress at that position.
+			as a length offset (in ''px'') from the scroll origin.
 			Null when the timeline is [=timeline inactive state|inactive=].
 	</dl>
 
-	Note: The {{ViewTimeline/startTime}} and {{ViewTimeline/endTime}} attributes
-	have odd names for being specific to ScrollTimeline;
-	this is to keep them consistent with {{AnimationTimeline/currentTime}},
-	which represents the currently-active position on the same scale.
+	Note: The value of {{ViewTimeline/startOffset}} can be greater than {{ViewTimeline/endOffset}},
+	for example in the [=horizontal axis=] in a right-to-left (''rtl'') [=writing mode=].
+
+	ISSUE: This makes the offsets compatible with the way scrollLeft and scrollTop work,
+	which go negative from zero in RTL scroll containers,
+	is this how we want it to work here?
 
 	Inherited attributes: