Add comment to explain callback ordering

components/script/dom/document.rs

@@ -1208,6 +1208,10 @@ impl Document {
             callback(*timing);
         }
 
+        // Only send the animation change state message after running any callbacks.
+        // This means that if the animation callback adds a new callback for
+        // the next frame (which is the common case), we won't send a NoAnimationCallbacksPresent
+        // message quickly followed by an AnimationCallbacksPresent message.
         if self.animation_frame_list.borrow().is_empty() {
             let ConstellationChan(ref chan) = self.window.constellation_chan();
             let event = ConstellationMsg::ChangeRunningAnimationsState(self.window.pipeline(),