Precisely define the navigation events

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -625,15 +625,16 @@ To create an instance of the {{NavigationEvent}} interface, use the {{Navigation
 passing an optional {{NavigationEventInit}} dictionary.
 
 <pre class=idl>
-[Constructor(DOMString type, optional NavigationEventInit eventInitDict)]
+[Exposed=Window,
+  Constructor (DOMString type, optional NavigationEventInit eventInitDict)]
 interface NavigationEvent : UIEvent {
-      readonly attribute SpatialNavigationDirection dir;
-      readonly attribute EventTarget? relatedTarget;
+    readonly attribute SpatialNavigationDirection dir;
+    readonly attribute EventTarget? relatedTarget;
 };
 
 dictionary NavigationEventInit : UIEventInit {
-      required SpatialNavigationDirection dir;
-      EventTarget? relatedTarget = null;
+    SpatialNavigationDirection dir;
+    EventTarget? relatedTarget = null;
 };
 </pre>