diff --git a/testing/web-platform/tests/interfaces/event-timing.idl b/testing/web-platform/tests/interfaces/event-timing.idl
index c6790e16e3db..e8475d31922f 100644
--- a/testing/web-platform/tests/interfaces/event-timing.idl
+++ b/testing/web-platform/tests/interfaces/event-timing.idl
@@ -8,6 +8,7 @@ interface PerformanceEventTiming : PerformanceEntry {
     readonly attribute DOMHighResTimeStamp processingStart;
     readonly attribute DOMHighResTimeStamp processingEnd;
     readonly attribute boolean cancelable;
+    [Default] object toJSON();
 };
 
 [Exposed=Window]