[scroll-animations-1] Clarify interaction with animation events. w3c#4324

Author: fantasai

scroll-animations-1/Overview.bs

@@ -37,6 +37,7 @@ spec:web-animations-1;
 		text:active duration
 		text:iteration count
 		text:iteration duration
+		text:finished play state
 spec:html;
 	type:dfn; for:/; text:browsing context
 	type:method; text:requestAnimationFrame()
@@ -732,7 +733,27 @@ spec: cssom-view-1; type: dfn;
 
 	ISSUE: Do we want to expand this scope to preceding siblings and/or distant cousins?
 
-# Avoiding cycles with layout # {#avoiding-cycles}
+## Animation Events ## {#events}
+
+	[=Scroll-driven animations=] dispatch all the same animation events
+	as the more typical time-driven animations
+	as described in
+	[[web-animations-1#animation-events-section]],
+	[[css-animations-1#events]],
+	and [[css-animations-2#event-dispatch]].
+
+	Note: When scrolling backwards,
+	the <code>animationstart</code> event will fire
+	at the <em>end</em> of the [=active interval=],
+	and the <code>animationend</code> event will fire
+	at the start of the [=active interval=].
+	However, since the <code>finish</code> event
+	is about entering the [=finished play state=],
+	it only fires when scrolling forwards.
+
+# Frame Calculation Details # {#frames}
+
+## Avoiding cycles with layout ## {#avoiding-cycles}
 
 	The ability for scrolling to drive the progress of an animation,
 	gives rise to the possibility of <dfn>layout cycles</dfn>,