diff --git a/testing/web-platform/tests/interfaces/layout-instability.idl b/testing/web-platform/tests/interfaces/layout-instability.idl
index 1bacea77bcef..c46151a923dd 100644
--- a/testing/web-platform/tests/interfaces/layout-instability.idl
+++ b/testing/web-platform/tests/interfaces/layout-instability.idl
@@ -7,4 +7,5 @@ interface LayoutShift : PerformanceEntry {
   readonly attribute long value;
   readonly attribute boolean hadRecentInput;
   readonly attribute DOMHighResTimeStamp lastInputTime;
+  [Default] object toJSON();
 };