[css-image-animation] Cross reference issue

Author: frivoal

css-image-animation-1/Overview.bs

@@ -391,7 +391,7 @@ Terminology {#terminology}
 			distinctly from the initial state of the animation.
 	</dl>
 
-	Issue: Should the <{video}> element
+	Issue(w3c/csswg-drafts#13641): Should the <{video}> element
 	when it represents its <a spec=html>poster frame</a>
 	be treated as a [=content image=],
 	a [=decorative image=],