[css-image-animation] Add note on propagation

See w3c#13435

Author: frivoal

css-image-animation-1/Overview.bs

@@ -108,6 +108,13 @@ Controlling Image Animations: the 'image-animation' property {#image-animation}
 	When this property is applied to the [=root element=],
 	its effects on background [=decorative images=] are propagated to the [=canvas background=].
 
+	Note: For legacy reasons,
+	the 'background' property propagates not only from the [=root element=],
+	but also from the HTML <{body}> element.
+	However, newer properties like 'image-animation' <em>only</em> propagate from the [=root element=].
+	Setting it on the <{body}> will not propagate,
+	even if that is where the 'background' is set.
+
 	<dl dfn-for=image-animation dfn-type=value>
 		<dt><dfn>normal</dfn>
 		<dd>