[scroll-animations-1][web-animations-2] Deferred start time (w3c#9181)

When using a scroll-driven animation the start time of the animation is auto-aligned with the animation range boundary when it has not been explicitly set.

Author: kevers-google

scroll-animations-1/Overview.bs

@@ -31,6 +31,8 @@ Markup Shorthands: markdown yes
 <pre class=anchors>
 urlPrefix: https://html.spec.whatwg.org/multipage/window-object.html; type: dfn; spec: html
 	text: document associated with a window; url: concept-document-window
+urlPrefix: https://drafts.csswg.org/web-animations-2/; type: dfn; spec: web-animations-2
+	text: calculating an auto-aligned start time
 </pre>
 <pre class=link-defaults>
 spec:web-animations-1;
@@ -950,6 +952,11 @@ spec:selectors-4; type:dfn; text:selector
 	and the state of a scroll-driven animation
 	may be inconsistent in the composited frame.
 
+	When updating timeline current time,
+	the [=start time=] of any attached animation is conditionally updated.
+	For each attached animation,
+	run the procedure for [=calculating an auto-aligned start time=].
+
 # Privacy Considerations # {#privacy-considerations}
 
 	There are no known privacy impacts of the features in this specification.