[css-animations-2] Specify that animation-timeline is applied first. w3c#5653

Author: fantasai

css-animations-2/Overview.bs

@@ -633,6 +633,10 @@ for both their keyframes and timeline which is simple and ergonomics. The
 'animation-timeline' property gives authors additional control to independently
 select keyframes and timeline if necessary.
 
+When multiple 'animation-*' properties are set simultaneously,
+'animation-timeline' is updated first,
+so e.g. a change to 'animation-play-state' applies
+to the simultaneously-applied timeline specified in 'animation-timeline'.
 
 ## The 'animation' shorthand property ## {#animation-shorthand}