diff --git a/testing/web-platform/tests/interfaces/pointerevents.idl b/testing/web-platform/tests/interfaces/pointerevents.idl
index da822bba0e58..fd368bf5a0a2 100644
--- a/testing/web-platform/tests/interfaces/pointerevents.idl
+++ b/testing/web-platform/tests/interfaces/pointerevents.idl
@@ -36,7 +36,7 @@ partial interface Element {
   boolean hasPointerCapture(long pointerId);
 };
 
-partial interface GlobalEventHandlers {
+partial interface mixin GlobalEventHandlers {
     attribute EventHandler ongotpointercapture;
     attribute EventHandler onlostpointercapture;
     attribute EventHandler onpointerdown;