[scroll-animations-1] Fix grammatical ambiguity, add subheadings.

Author: fantasai

scroll-animations-1/Overview.bs

@@ -873,22 +873,28 @@ spec: cssom-view-1; type: dfn;
 	                         ╶┄┄┄┄┄ duration┄┄┄┄┄╴
 	</pre>
 
+	ISSUE: Define application to time-driven animations.
+
+### Specifying an Animation’s Timeline Range: the 'animation-range' shorthand ### {#animation-range}
+
 	<pre class="propdef shorthand">
 		Name: animation-range
-		Value: [ <<'animation-range-start'>> <<'animation-range-end'>>? | <<timeline-range-name>> ]#
+		Value: [ <<'animation-range-start'>> <<'animation-range-end'>>? ]#
 	</pre>
 
 	The 'animation-range' property is a [=shorthand property|shorthand=]
 	that sets 'animation-range-start' and 'animation-range-end'
 	together in a single declaration.
-	If the <<'animation-range-end'>> value is omitted,
-	it is set to ''animation-range-end/normal''.
 	If a <<timeline-range-name>> alone is specified,
 	'animation-range-start' is set to that name plus ''0%''
 	and 'animation-range-end' is set to that name plus ''100%''.
+	Otherwise, if the <<'animation-range-end'>> value is omitted,
+	it is set to ''animation-range-end/normal''.
 
 	ISSUE(8438): What's the best way to handle defaulting of omitted values here?
 
+### Specifying an Animation’s Timeline Range Start: the 'animation-range-start' property ### {#animation-range-start}
+
 	<pre class="propdef">
 		Name: animation-range-start
 		Value: [ normal | <<timeline-range-name>> <<percentage>> ]#
@@ -918,6 +924,8 @@ spec: cssom-view-1; type: dfn;
 			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>> ]#