diff --git a/testing/web-platform/tests/interfaces/speech-api.idl b/testing/web-platform/tests/interfaces/speech-api.idl
index f94637a8d21e..ba86d5065b7a 100644
--- a/testing/web-platform/tests/interfaces/speech-api.idl
+++ b/testing/web-platform/tests/interfaces/speech-api.idl
@@ -155,6 +155,7 @@ interface SpeechSynthesisUtterance : EventTarget {
 interface SpeechSynthesisEvent : Event {
     readonly attribute SpeechSynthesisUtterance utterance;
     readonly attribute unsigned long charIndex;
+    readonly attribute unsigned long charLength;
     readonly attribute float elapsedTime;
     readonly attribute DOMString name;
 };
@@ -162,6 +163,7 @@ interface SpeechSynthesisEvent : Event {
 dictionary SpeechSynthesisEventInit : EventInit {
     required SpeechSynthesisUtterance utterance;
     unsigned long charIndex = 0;
+    unsigned long charLength = 0;
     float elapsedTime = 0;
     DOMString name = "";
 };