--- a/testing/web-platform/tests/interfaces/dom.idl
+++ b/testing/web-platform/tests/interfaces/dom.idl
@@ -1,14 +1,14 @@
 // GENERATED CONTENT - DO NOT EDIT
 // Content was automatically extracted by Reffy into reffy-reports
 // (https://github.com/tidoust/reffy-reports)
 // Source: DOM Standard (https://dom.spec.whatwg.org/)
 
-[Constructor(DOMString type, optional EventInit eventInitDict),
+[Constructor(DOMString type, optional EventInit eventInitDict = {}),
  Exposed=(Window,Worker,AudioWorklet)]
 interface Event {
   readonly attribute DOMString type;
   readonly attribute EventTarget? target;
   readonly attribute EventTarget? srcElement; // historical
   readonly attribute EventTarget? currentTarget;
   sequence<EventTarget> composedPath();
 
@@ -40,33 +40,33 @@ dictionary EventInit {
   boolean cancelable = false;
   boolean composed = false;
 };
 
 partial interface Window {
   [Replaceable] readonly attribute any event; // historical
 };
 
-[Constructor(DOMString type, optional CustomEventInit eventInitDict),
+[Constructor(DOMString type, optional CustomEventInit eventInitDict = {}),
  Exposed=(Window,Worker)]
 interface CustomEvent : Event {
   readonly attribute any detail;
 
   void initCustomEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false, optional any detail = null);
 };
 
 dictionary CustomEventInit : EventInit {
   any detail = null;
 };
 
 [Constructor,
  Exposed=(Window,Worker,AudioWorklet)]
 interface EventTarget {
-  void addEventListener(DOMString type, EventListener? callback, optional (AddEventListenerOptions or boolean) options);
-  void removeEventListener(DOMString type, EventListener? callback, optional (EventListenerOptions or boolean) options);
+  void addEventListener(DOMString type, EventListener? callback, optional (AddEventListenerOptions or boolean) options = {});
+  void removeEventListener(DOMString type, EventListener? callback, optional (EventListenerOptions or boolean) options = {});
   boolean dispatchEvent(Event event);
 };
 
 callback interface EventListener {
   void handleEvent(Event event);
 };
 
 dictionary EventListenerOptions {
@@ -153,17 +153,17 @@ interface HTMLCollection {
   readonly attribute unsigned long length;
   getter Element? item(unsigned long index);
   getter Element? namedItem(DOMString name);
 };
 
 [Constructor(MutationCallback callback),
  Exposed=Window]
 interface MutationObserver {
-  void observe(Node target, optional MutationObserverInit options);
+  void observe(Node target, optional MutationObserverInit options = {});
   void disconnect();
   sequence<MutationRecord> takeRecords();
 };
 
 callback MutationCallback = void (sequence<MutationRecord> mutations, MutationObserver observer);
 
 dictionary MutationObserverInit {
   boolean childList = false;
@@ -204,17 +204,17 @@ interface Node : EventTarget {
   const unsigned short NOTATION_NODE = 12; // historical
   readonly attribute unsigned short nodeType;
   readonly attribute DOMString nodeName;
 
   readonly attribute USVString baseURI;
 
   readonly attribute boolean isConnected;
   readonly attribute Document? ownerDocument;
-  Node getRootNode(optional GetRootNodeOptions options);
+  Node getRootNode(optional GetRootNodeOptions options = {});
   readonly attribute Node? parentNode;
   readonly attribute Element? parentElement;
   boolean hasChildNodes();
   [SameObject] readonly attribute NodeList childNodes;
   readonly attribute Node? firstChild;
   readonly attribute Node? lastChild;
   readonly attribute Node? previousSibling;
   readonly attribute Node? nextSibling;
@@ -264,18 +264,18 @@ interface Document : Node {
   readonly attribute DOMString contentType;
 
   readonly attribute DocumentType? doctype;
   readonly attribute Element? documentElement;
   HTMLCollection getElementsByTagName(DOMString qualifiedName);
   HTMLCollection getElementsByTagNameNS(DOMString? namespace, DOMString localName);
   HTMLCollection getElementsByClassName(DOMString classNames);
 
-  [CEReactions, NewObject] Element createElement(DOMString localName, optional (DOMString or ElementCreationOptions) options);
-  [CEReactions, NewObject] Element createElementNS(DOMString? namespace, DOMString qualifiedName, optional (DOMString or ElementCreationOptions) options);
+  [CEReactions, NewObject] Element createElement(DOMString localName, optional (DOMString or ElementCreationOptions) options = {});
+  [CEReactions, NewObject] Element createElementNS(DOMString? namespace, DOMString qualifiedName, optional (DOMString or ElementCreationOptions) options = {});
   [NewObject] DocumentFragment createDocumentFragment();
   [NewObject] Text createTextNode(DOMString data);
   [NewObject] CDATASection createCDATASection(DOMString data);
   [NewObject] Comment createComment(DOMString data);
   [NewObject] ProcessingInstruction createProcessingInstruction(DOMString target, DOMString data);
 
   [CEReactions, NewObject] Node importNode(Node node, optional boolean deep = false);
   [CEReactions] Node adoptNode(Node node);
@@ -354,17 +354,17 @@ interface Element : Node {
   boolean hasAttributeNS(DOMString? namespace, DOMString localName);
 
   Attr? getAttributeNode(DOMString qualifiedName);
   Attr? getAttributeNodeNS(DOMString? namespace, DOMString localName);
   [CEReactions] Attr? setAttributeNode(Attr attr);
   [CEReactions] Attr? setAttributeNodeNS(Attr attr);
   [CEReactions] Attr removeAttributeNode(Attr attr);
 
-  ShadowRoot attachShadow(ShadowRootInit init);
+  ShadowRoot attachShadow(optional ShadowRootInit init = {});
   readonly attribute ShadowRoot? shadowRoot;
 
   Element? closest(DOMString selectors);
   boolean matches(DOMString selectors);
   boolean webkitMatchesSelector(DOMString selectors); // historical alias of .matches
 
   HTMLCollection getElementsByTagName(DOMString qualifiedName);
   HTMLCollection getElementsByTagNameNS(DOMString? namespace, DOMString localName);