diff --git a/dom.bs b/dom.bs
index ece27206..e3aa5d45 100644
--- a/dom.bs
+++ b/dom.bs
@@ -3209,7 +3209,8 @@ removed so they do not get lost when <code>subtree</code> is set to true on <a>n
 <h4 id=interface-mutationobserver>Interface {{MutationObserver}}</h4>
 
 <pre class="idl">
-[Constructor(MutationCallback callback)]
+[Constructor(MutationCallback callback),
+ Exposed=Window]
 interface MutationObserver {
   void observe(Node target, optional MutationObserverInit options);
   void disconnect();