[css-image-animation] Define propagation to the canvas (#13640)

Addresses #13435

Author: frivoal

css-image-animation-1/Overview.bs

@@ -105,6 +105,9 @@ Controlling Image Animations: the 'image-animation' property {#image-animation}
 	the different values of this property have no effect.
 	This property does not affect [=videos=] nor [=programmatic images=] either.
 
+	When this property is applied to the [=root element=],
+	its effects on background [=decorative images=] are propagated to the [=canvas background=].
+
 	<dl dfn-for=image-animation dfn-type=value>
 		<dt><dfn>normal</dfn>
 		<dd>