[web-animations-1] Require a style flush at the end of commitStyles

Fixes #11084.

Author: birtles

web-animations-1/Overview.bs

@@ -4877,6 +4877,22 @@ The {{Animation}} interface {#the-animation-interface}
       calling this method <em>does</em> trigger a [=style change event=]
       (see [[#model-liveness]]).
 
+      Furthermore,
+      after updating the style attribute of each effect target,
+      the user agent must ensure that the corresponding computed style
+      is updated.
+
+      <div class=note>
+
+        Without the requirement to update the computed style
+        at the end of the procedure,
+        calling `commitStyles` followed by immediately canceling an animation
+        may cause a transition to be generated since the changes to the
+        style attributes would not be reflected in the
+        [=before-change style=] during the next [=style change event=].
+
+      </div>
+
       <div class=note>
 
         Since the procedure to [=commit computed styles=]