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diff --git a/dom/animation/ComputedTiming.h b/dom/animation/ComputedTiming.h
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+++ b/dom/animation/ComputedTiming.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_ComputedTiming_h
+#define mozilla_ComputedTiming_h
+
+#include "mozilla/dom/Nullable.h"
+#include "mozilla/StickyTimeDuration.h"
+
+#include "mozilla/dom/AnimationEffectBinding.h"  // FillMode
+
+namespace mozilla {
+
+/**
+ * Stores the results of calculating the timing properties of an animation
+ * at a given sample time.
+ */
+struct ComputedTiming {
+  // The total duration of the animation including all iterations.
+  // Will equal StickyTimeDuration::Forever() if the animation repeats
+  // indefinitely.
+  StickyTimeDuration mActiveDuration;
+  // The time within the active interval.
+  StickyTimeDuration mActiveTime;
+  // The effect end time in local time (i.e. an offset from the effect's
+  // start time). Will equal StickyTimeDuration::Forever() if the animation
+  // plays indefinitely.
+  StickyTimeDuration mEndTime;
+  // Progress towards the end of the current iteration. If the effect is
+  // being sampled backwards, this will go from 1.0 to 0.0.
+  // Will be null if the animation is neither animating nor
+  // filling at the sampled time.
+  dom::Nullable<double> mProgress;
+  // Zero-based iteration index (meaningless if mProgress is null).
+  uint64_t mCurrentIteration = 0;
+  // Unlike TimingParams::mIterations, this value is
+  // guaranteed to be in the range [0, Infinity].
+  double mIterations = 1.0;
+  double mIterationStart = 0.0;
+  StickyTimeDuration mDuration;
+
+  // This is the computed fill mode so it is never auto
+  dom::FillMode mFill = dom::FillMode::None;
+  bool FillsForwards() const {
+    MOZ_ASSERT(mFill != dom::FillMode::Auto,
+               "mFill should not be Auto in ComputedTiming.");
+    return mFill == dom::FillMode::Both || mFill == dom::FillMode::Forwards;
+  }
+  bool FillsBackwards() const {
+    MOZ_ASSERT(mFill != dom::FillMode::Auto,
+               "mFill should not be Auto in ComputedTiming.");
+    return mFill == dom::FillMode::Both || mFill == dom::FillMode::Backwards;
+  }
+
+  enum class AnimationPhase {
+    Idle,    // Not sampled (null sample time)
+    Before,  // Sampled prior to the start of the active interval
+    Active,  // Sampled within the active interval
+    After    // Sampled after (or at) the end of the active interval
+  };
+  AnimationPhase mPhase = AnimationPhase::Idle;
+
+  bool mBeforeFlag = false;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ComputedTiming_h