[css-animations-2] Define order for sorting animationcancel events

See #11064.

css-animations-2/Overview.bs

@@ -234,6 +234,18 @@ last) as follows:
     computed value of the 'animation-name' property of the (common) <a>owning
     element</a>.
 
+When determining the composite order in order
+to sort [[#events|animation events]]
+where either or both of the events is an {{animationcancel}} event,
+treat the CSS Animation(s) for which the {{animationcancel}} event was generated
+as having an [=owning element=] corresponding to
+the owning element in use at the moment when the CSS Animation was cancelled.
+Furthermore,
+use the position of the animation in the 'animation-name' property
+in effect at the time when the CSS Animation was cancelled
+sorting such that positions of cancelled animations sort
+before positions of animations that have not been cancelled.
+
 The composite order of CSS Animations <em>without</em> an <a>owning element</a>
 is based on their position in the <a>global animation list</a>.