diff --git a/testing/web-platform/tests/interfaces/web-animations.idl b/testing/web-platform/tests/interfaces/web-animations.idl
index 64265c3bf3fc..ae64bb867eb2 100644
--- a/testing/web-platform/tests/interfaces/web-animations.idl
+++ b/testing/web-platform/tests/interfaces/web-animations.idl
@@ -6,6 +6,7 @@
 [Exposed=Window]
 interface AnimationTimeline {
     readonly attribute double? currentTime;
+    readonly attribute TimelinePhase phase;
 };
 
 dictionary DocumentTimelineOptions {
@@ -49,6 +50,8 @@ enum AnimationPlayState { "idle", "running", "paused", "finished" };
 
 enum AnimationReplaceState { "active", "removed", "persisted" };
 
+enum TimelinePhase { "inactive", "before", "active", "after" };
+
 [Exposed=Window]
 interface AnimationEffect {
     EffectTiming         getTiming();