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diff --git a/dom/animation/AnimationTimeline.h b/dom/animation/AnimationTimeline.h
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+++ b/dom/animation/AnimationTimeline.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_AnimationTimeline_h
+#define mozilla_dom_AnimationTimeline_h
+
+#include "nsISupports.h"
+#include "nsWrapperCache.h"
+#include "nsCycleCollectionParticipant.h"
+#include "mozilla/AnimationUtils.h"
+#include "nsHashKeys.h"
+#include "nsIGlobalObject.h"
+#include "nsTHashSet.h"
+
+namespace mozilla::dom {
+
+class Animation;
+class Document;
+class ScrollTimeline;
+
+class AnimationTimeline : public nsISupports, public nsWrapperCache {
+ public:
+  explicit AnimationTimeline(nsIGlobalObject* aWindow,
+                             RTPCallerType aRTPCallerType);
+
+  struct TickState {
+    TickState() = default;
+    // Nothing here, but this might be useful in the future.
+  };
+
+ protected:
+  virtual ~AnimationTimeline();
+
+  // Tick animations and may remove them from the list if we don't need to tick
+  // it. Return true if any animations need to be ticked.
+  bool Tick(TickState&);
+
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(AnimationTimeline)
+
+  nsIGlobalObject* GetParentObject() const { return mWindow; }
+
+  // AnimationTimeline methods
+  virtual Nullable<TimeDuration> GetCurrentTimeAsDuration() const = 0;
+
+  // Wrapper functions for AnimationTimeline DOM methods when called from
+  // script.
+  Nullable<double> GetCurrentTimeAsDouble() const {
+    return AnimationUtils::TimeDurationToDouble(GetCurrentTimeAsDuration(),
+                                                mRTPCallerType);
+  }
+
+  TimeStamp GetCurrentTimeAsTimeStamp() const {
+    Nullable<TimeDuration> currentTime = GetCurrentTimeAsDuration();
+    return !currentTime.IsNull() ? ToTimeStamp(currentTime.Value())
+                                 : TimeStamp();
+  }
+
+  /**
+   * Returns true if the times returned by GetCurrentTimeAsDuration() are
+   * convertible to and from wallclock-based TimeStamp (e.g. from
+   * TimeStamp::Now()) values using ToTimelineTime() and ToTimeStamp().
+   *
+   * Typically this is true, but it will be false in the case when this
+   * timeline has no refresh driver or is tied to a refresh driver under test
+   * control.
+   */
+  virtual bool TracksWallclockTime() const = 0;
+
+  /**
+   * Converts a TimeStamp to the equivalent value in timeline time.
+   * Note that when TracksWallclockTime() is false, there is no correspondence
+   * between timeline time and wallclock time. In such a case, passing a
+   * timestamp from TimeStamp::Now() to this method will not return a
+   * meaningful result.
+   */
+  virtual Nullable<TimeDuration> ToTimelineTime(
+      const TimeStamp& aTimeStamp) const = 0;
+
+  virtual TimeStamp ToTimeStamp(const TimeDuration& aTimelineTime) const = 0;
+
+  /**
+   * Inform this timeline that |aAnimation| which is or was observing the
+   * timeline, has been updated. This serves as both the means to associate
+   * AND disassociate animations with a timeline. The timeline itself will
+   * determine if it needs to begin, continue or stop tracking this animation.
+   */
+  virtual void NotifyAnimationUpdated(Animation& aAnimation);
+
+  /**
+   * Returns true if any CSS animations, CSS transitions or Web animations are
+   * currently associated with this timeline.  As soon as an animation is
+   * applied to an element it is associated with the timeline even if it has a
+   * delayed start, so this includes animations that may not be active for some
+   * time.
+   */
+  bool HasAnimations() const { return !mAnimations.IsEmpty(); }
+
+  virtual void RemoveAnimation(Animation* aAnimation);
+  virtual void NotifyAnimationContentVisibilityChanged(Animation* aAnimation,
+                                                       bool aIsVisible);
+  void UpdateHiddenByContentVisibility();
+
+  virtual Document* GetDocument() const = 0;
+
+  virtual bool IsMonotonicallyIncreasing() const = 0;
+
+  RTPCallerType GetRTPCallerType() const { return mRTPCallerType; }
+
+  virtual bool IsScrollTimeline() const { return false; }
+  virtual const ScrollTimeline* AsScrollTimeline() const { return nullptr; }
+  virtual bool IsViewTimeline() const { return false; }
+
+  // For a monotonic timeline, there is no upper bound on current time, and
+  // timeline duration is unresolved. For a non-monotonic (e.g. scroll)
+  // timeline, the duration has a fixed upper bound.
+  //
+  // https://drafts.csswg.org/web-animations-2/#timeline-duration
+  virtual Nullable<TimeDuration> TimelineDuration() const { return nullptr; }
+
+ protected:
+  nsCOMPtr<nsIGlobalObject> mWindow;
+
+  // Animations observing this timeline
+  //
+  // We store them in (a) a hashset for quick lookup, and (b) a LinkedList to
+  // maintain a fixed sampling order. Animations that are hidden by
+  // `content-visibility` are not sampled and will only be in the hashset.
+  // The LinkedList should always be a subset of the hashset.
+  //
+  // The hashset keeps a strong reference to each animation since
+  // dealing with addref/release with LinkedList is difficult.
+  using AnimationSet = nsTHashSet<nsRefPtrHashKey<dom::Animation>>;
+  AnimationSet mAnimations;
+  LinkedList<dom::Animation> mAnimationOrder;
+
+  // Whether the Timeline is System, ResistFingerprinting, or neither
+  enum RTPCallerType mRTPCallerType;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_AnimationTimeline_h