[css-nav-1] Align with Web IDL specification (w3c#4798)

Author: autokagami

css-nav-1/Overview.bs

@@ -491,8 +491,8 @@ dictionary SpatialNavigationSearchOptions {
 
 partial interface Element {
     Node getSpatialNavigationContainer();
-    sequence<Node> focusableAreas(optional FocusableAreasOption option);
-    Node? spatialNavigationSearch(SpatialNavigationDirection dir, optional SpatialNavigationSearchOptions options);
+    sequence<Node> focusableAreas(optional FocusableAreasOption option = {});
+    Node? spatialNavigationSearch(SpatialNavigationDirection dir, optional SpatialNavigationSearchOptions options = {});
 };
 </pre>
 
@@ -594,7 +594,7 @@ passing an optional {{NavigationEventInit}} dictionary.
 [Exposed=Window]
 interface NavigationEvent : UIEvent {
     constructor(DOMString type,
-                optional NavigationEventInit eventInitDict);
+                optional NavigationEventInit eventInitDict = {});
     readonly attribute SpatialNavigationDirection dir;
     readonly attribute EventTarget? relatedTarget;
 };