[web-animations-1] Array-form keyframes argument can include null and undefined

Author: cdoublev

web-animations-1/Overview.bs

@@ -6100,7 +6100,7 @@ The {{KeyframeEffect}} interface {#the-keyframeeffect-interface}
       (CompositeOperationOrAuto or sequence<CompositeOperationOrAuto>) composite = [];
     };
 
-    typedef (sequence<Keyframe> or PropertyIndexedKeyframes) KeyframeArgument;
+    typedef (sequence<Keyframe?> or PropertyIndexedKeyframes) KeyframeArgument;
   </xmp>
 
   The meaning and allowed values of each argument is as follows: