[scroll-animations-1] Add self keyword

#8227 (comment)

Author: bramus

scroll-animations-1/Overview.bs

@@ -144,7 +144,7 @@ spec: cssom-view-1; type: dfn;
 	<pre class="prod">
 		<<scroll()>> = scroll( [ <<scroller>> || <<axis>> ]? )
 		<dfn noexport><<axis>></dfn> = block | inline | vertical | horizontal
-		<dfn noexport><<scroller>></dfn> = root | nearest
+		<dfn noexport><<scroller>></dfn> = root | nearest | self
 	</pre>
 
 	By default,
@@ -181,6 +181,10 @@ spec: cssom-view-1; type: dfn;
 		<dt><dfn>root</dfn>
 		<dd>
 			Specifies to use the document viewport as the [=scroll container=].
+
+		<dt><dfn>self</dfn>
+		<dd>
+			Specifies to use the element’s own box as the [=scroll container=].
 	</dl>
 
 ### The {{ScrollTimeline}} Interface ### {#scrolltimeline-interface}