[scroll-animations-1] Allow getCurrentTime to return out-of-range times. w3c#8114

Author: fantasai

scroll-animations-1/Overview.bs

@@ -898,11 +898,16 @@ spec: cssom-view-1; type: dfn;
 
 			1. If |rangeName| is a valid [=named timeline range=] on [=this=],
 				let |progress| be the current progress through that range,
-				expressed as a percentage value between 0 and 100.
+				expressed as a percentage value.
 
 				Create a [=new unit value=] from (|progress|, "percent")
 				and return it.
 
+				If the start and end points of the [=named timeline range=] coincide,
+				return negative infinity for time values before it,
+				positive infinity for time values after it,
+				and zero for time values coinciding with that point.
+
 			2. Otherwise, return null.
 
 		2. Let |current time|