[scroll-animations-1] Add length offsets to animation-range w3c#7575

* Add <length> offsets to timeline range offsets
* Add <length-percentage> values referencing the whole timeline
* Tighten up definitions

Author: fantasai

scroll-animations-1/Overview.bs

@@ -897,11 +897,11 @@ spec: cssom-view-1; type: dfn;
 
 	<pre class="propdef">
 		Name: animation-range-start
-		Value: [ normal | <<timeline-range-name>> <<percentage>> ]#
+		Value: [ normal | <<length-percentage>> | <<timeline-range-name>> <<length-percentage>>? ]#
 		Initial: normal
 		Applies to: all elements
 		Inherited: no
-		Percentages: relative to the specified [=named timeline range=]
+		Percentages: relative to the specified [=named timeline range=] if one was specified, else to the entire timeline
 		Computed value: list, each item either the keyword ''animation-range-start/normal'' or a timeline range and progress percentage
 		Animation type: not animatable
 	</pre>
@@ -917,22 +917,30 @@ spec: cssom-view-1; type: dfn;
 			The start of the animation’s [=active interval=]
 			is determined as normal.
 
-		<dt><dfn><<timeline-range-name>> <<percentage>></dfn>
+		<dt><dfn><<length-percentage>></dfn>
+		<dd>
+			The [=active interval=] starts
+			at the specified point on the [=timeline=]
+			measuring from the start of the timeline,
+			plus any applicable [=start delay=].
+
+		<dt><dfn><<timeline-range-name>> <<length-percentage>>?</dfn>
 		<dd>
 			The [=active interval=] starts
-			at the specified point on the [=timeline=],
+			at the specified point on the [=timeline=]
+			measuring from the start of the specified [=named timeline range=],
 			plus any applicable [=start delay=].
 	</dl>
 
 ### Specifying an Animation’s Timeline Range End: the 'animation-range-end' property ### {#animation-range-end}
 
 	<pre class="propdef">
 		Name: animation-range-end
-		Value: [ normal | <<timeline-range-name>> <<percentage>> ]#
+		Value: [ normal | <<length-percentage>> | <<timeline-range-name>> <<length-percentage>>? ]#
 		Initial: normal
 		Applies to: all elements
 		Inherited: no
-		Percentages: relative to the specified [=named timeline range=]
+		Percentages: relative to the specified [=named timeline range=] if one was specified, else to the entire timeline
 		Computed value: list, each item either the keyword ''animation-range-end/normal'' or a timeline range and progress percentage
 		Animation type: not animatable
 	</pre>
@@ -948,13 +956,19 @@ spec: cssom-view-1; type: dfn;
 			The end of the animation’s [=active interval=]
 			is determined as normal.
 
-		<dt><dfn><<timeline-range-name>> <<percentage>></dfn>
+		<dt><dfn><<length-percentage>></dfn>
+		<dd>
+			The [=active interval=] ends
+			at the specified point on the [=timeline=]
+			measuring from the start of the timeline,
+			minus any applicable [=end delay=].
+
+		<dt><dfn><<timeline-range-name>> <<length-percentage>>?</dfn>
 		<dd>
-			The end of the [=active interval=]
-			is at the specified point on the [=timeline=]
-			minus any applicable [=end delay=],
-			or at the end of the animation duration,
-			whichever comes first.
+			The [=active interval=] ends
+			at the specified point on the [=timeline=]
+			measuring from the start of the specified [=named timeline range=],
+			minus any applicable [=end delay=].
 	</dl>
 
 ## Reporting Timeline Range Progress: the ''getCurrentTime()'' method ## {#named-range-get-time}