Convert Region to a mixin

Following whatwg/webidl#433.

Author: foolip

css-regions-1/Overview.bs

@@ -1165,26 +1165,23 @@ The NamedFlow interface</h3>
 	to the <code>NULL</code> state.
 
 <h3 id="the-region-interface">
-The Region interface</h3>
+The Region mixin</h3>
 
-	The {{Region}}
-	interface is a
-	<a href="https://www.w3.org/TR/WebIDL/#idl-implements-statements">supplemental interface</a>
-	which must be implemented by all objects
+	{{Region}} is a
+	<a href="https://heycam.github.io/webidl/#idl-interface-mixins">mixin</a>
+	which must be included by all interfaces
 	(<a href="https://www.w3.org/TR/dom/#interface-element">
 		<code class= "idl">Elements</code></a>,
 	pseudo-elements or other CSS constructs
 	such as <a href="https://drafts.csswg.org/css3-page-template/#templates-and-slots">slots</a>) in an implementation which can be <a>CSS Regions</a>.
 
 	<pre class="idl">
-		[Exposed=Window,
-		 NoInterfaceObject]
 		interface Region {
 			readonly attribute CSSOMString regionOverset;
 			sequence&lt;Range&gt;? getRegionFlowRanges();
 		};
 
-		Element implements Region;
+		Element includes Region;
 	</pre>
 
 	The <dfn attribute for="Region"><code class= "idl">regionOverset</code></dfn>