Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1201 insertions, 0 deletions

diff --git a/dom/animation/KeyframeUtils.cpp b/dom/animation/KeyframeUtils.cpp
new file mode 100644
index 0000000000..329273fd22
--- /dev/null
+++ b/dom/animation/KeyframeUtils.cpp
@@ -0,0 +1,1201 @@
+/* -*- 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/. */
+
+#include "mozilla/KeyframeUtils.h"
+
+#include <algorithm>  // For std::stable_sort, std::min
+#include <utility>
+
+#include "jsapi.h"             // For most JSAPI
+#include "js/ForOfIterator.h"  // For JS::ForOfIterator
+#include "js/PropertyAndElement.h"  // JS_Enumerate, JS_GetProperty, JS_GetPropertyById
+#include "mozilla/AnimatedPropertyID.h"
+#include "mozilla/ComputedStyle.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/RangedArray.h"
+#include "mozilla/ServoBindingTypes.h"
+#include "mozilla/ServoBindings.h"
+#include "mozilla/ServoCSSParser.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/StyleAnimationValue.h"
+#include "mozilla/TimingParams.h"
+#include "mozilla/dom/BaseKeyframeTypesBinding.h"  // For FastBaseKeyframe etc.
+#include "mozilla/dom/BindingCallContext.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/KeyframeEffect.h"  // For PropertyValuesPair etc.
+#include "mozilla/dom/KeyframeEffectBinding.h"
+#include "mozilla/dom/Nullable.h"
+#include "nsCSSPropertyIDSet.h"
+#include "nsCSSProps.h"
+#include "nsCSSPseudoElements.h"  // For PseudoStyleType
+#include "nsClassHashtable.h"
+#include "nsContentUtils.h"  // For GetContextForContent
+#include "nsIScriptError.h"
+#include "nsPresContextInlines.h"
+#include "nsString.h"
+#include "nsTArray.h"
+
+using mozilla::dom::Nullable;
+
+namespace mozilla {
+
+// ------------------------------------------------------------------
+//
+// Internal data types
+//
+// ------------------------------------------------------------------
+
+// For the aAllowList parameter of AppendStringOrStringSequence and
+// GetPropertyValuesPairs.
+enum class ListAllowance { eDisallow, eAllow };
+
+/**
+ * A property-values pair obtained from the open-ended properties
+ * discovered on a regular keyframe or property-indexed keyframe object.
+ *
+ * Single values (as required by a regular keyframe, and as also supported
+ * on property-indexed keyframes) are stored as the only element in
+ * mValues.
+ */
+struct PropertyValuesPair {
+  PropertyValuesPair() : mProperty(eCSSProperty_UNKNOWN) {}
+
+  AnimatedPropertyID mProperty;
+  nsTArray<nsCString> mValues;
+};
+
+/**
+ * An additional property (for a property-values pair) found on a
+ * BaseKeyframe or BasePropertyIndexedKeyframe object.
+ */
+struct AdditionalProperty {
+  AnimatedPropertyID mProperty;
+  size_t mJsidIndex = 0;  // Index into |ids| in GetPropertyValuesPairs.
+
+  struct PropertyComparator {
+    bool Equals(const AdditionalProperty& aLhs,
+                const AdditionalProperty& aRhs) const {
+      return aLhs.mProperty == aRhs.mProperty;
+    }
+    bool LessThan(const AdditionalProperty& aLhs,
+                  const AdditionalProperty& aRhs) const {
+      bool customLhs =
+          aLhs.mProperty.mID == nsCSSPropertyID::eCSSPropertyExtra_variable;
+      bool customRhs =
+          aRhs.mProperty.mID == nsCSSPropertyID::eCSSPropertyExtra_variable;
+      if (!customLhs && !customRhs) {
+        // Compare by IDL names.
+        return nsCSSProps::PropertyIDLNameSortPosition(aLhs.mProperty.mID) <
+               nsCSSProps::PropertyIDLNameSortPosition(aRhs.mProperty.mID);
+      }
+      if (customLhs && customRhs) {
+        // Compare by custom property names.
+        return nsDependentAtomString(aLhs.mProperty.mCustomName) <
+               nsDependentAtomString(aRhs.mProperty.mCustomName);
+      }
+      // Custom properties should be ordered before normal CSS properties, as if
+      // the custom property name starts with `--`.
+      // <https://drafts.csswg.org/web-animations-1/#idl-attribute-name-to-animation-property-name>
+      return !customLhs && customRhs;
+    }
+  };
+};
+
+/**
+ * Data for a segment in a keyframe animation of a given property
+ * whose value is a StyleAnimationValue.
+ *
+ * KeyframeValueEntry is used in GetAnimationPropertiesFromKeyframes
+ * to gather data for each individual segment.
+ */
+struct KeyframeValueEntry {
+  KeyframeValueEntry()
+      : mProperty(eCSSProperty_UNKNOWN), mOffset(), mComposite() {}
+
+  AnimatedPropertyID mProperty;
+  AnimationValue mValue;
+
+  float mOffset;
+  Maybe<StyleComputedTimingFunction> mTimingFunction;
+  dom::CompositeOperation mComposite;
+
+  struct PropertyOffsetComparator {
+    static bool Equals(const KeyframeValueEntry& aLhs,
+                       const KeyframeValueEntry& aRhs) {
+      return aLhs.mProperty == aRhs.mProperty && aLhs.mOffset == aRhs.mOffset;
+    }
+    static bool LessThan(const KeyframeValueEntry& aLhs,
+                         const KeyframeValueEntry& aRhs) {
+      // First, sort by property name.
+      bool customLhs =
+          aLhs.mProperty.mID == nsCSSPropertyID::eCSSPropertyExtra_variable;
+      bool customRhs =
+          aRhs.mProperty.mID == nsCSSPropertyID::eCSSPropertyExtra_variable;
+      if (!customLhs && !customRhs) {
+        // Compare by IDL names.
+        int32_t order =
+            nsCSSProps::PropertyIDLNameSortPosition(aLhs.mProperty.mID) -
+            nsCSSProps::PropertyIDLNameSortPosition(aRhs.mProperty.mID);
+        if (order != 0) {
+          return order < 0;
+        }
+      } else if (customLhs && customRhs) {
+        // Compare by custom property names.
+        int order = Compare(nsDependentAtomString(aLhs.mProperty.mCustomName),
+                            nsDependentAtomString(aRhs.mProperty.mCustomName));
+        if (order != 0) {
+          return order < 0;
+        }
+      } else {
+        return !customLhs && customRhs;
+      }
+
+      // Then, by offset.
+      return aLhs.mOffset < aRhs.mOffset;
+    }
+  };
+};
+
+class ComputedOffsetComparator {
+ public:
+  static bool Equals(const Keyframe& aLhs, const Keyframe& aRhs) {
+    return aLhs.mComputedOffset == aRhs.mComputedOffset;
+  }
+
+  static bool LessThan(const Keyframe& aLhs, const Keyframe& aRhs) {
+    return aLhs.mComputedOffset < aRhs.mComputedOffset;
+  }
+};
+
+// ------------------------------------------------------------------
+//
+// Internal helper method declarations
+//
+// ------------------------------------------------------------------
+
+static void GetKeyframeListFromKeyframeSequence(
+    JSContext* aCx, dom::Document* aDocument, JS::ForOfIterator& aIterator,
+    nsTArray<Keyframe>& aResult, const char* aContext, ErrorResult& aRv);
+
+static bool ConvertKeyframeSequence(JSContext* aCx, dom::Document* aDocument,
+                                    JS::ForOfIterator& aIterator,
+                                    const char* aContext,
+                                    nsTArray<Keyframe>& aResult);
+
+static bool GetPropertyValuesPairs(JSContext* aCx,
+                                   JS::Handle<JSObject*> aObject,
+                                   ListAllowance aAllowLists,
+                                   nsTArray<PropertyValuesPair>& aResult);
+
+static bool AppendStringOrStringSequenceToArray(JSContext* aCx,
+                                                JS::Handle<JS::Value> aValue,
+                                                ListAllowance aAllowLists,
+                                                nsTArray<nsCString>& aValues);
+
+static bool AppendValueAsString(JSContext* aCx, nsTArray<nsCString>& aValues,
+                                JS::Handle<JS::Value> aValue);
+
+static Maybe<PropertyValuePair> MakePropertyValuePair(
+    const AnimatedPropertyID& aProperty, const nsACString& aStringValue,
+    dom::Document* aDocument);
+
+static bool HasValidOffsets(const nsTArray<Keyframe>& aKeyframes);
+
+#ifdef DEBUG
+static void MarkAsComputeValuesFailureKey(PropertyValuePair& aPair);
+
+#endif
+
+static nsTArray<ComputedKeyframeValues> GetComputedKeyframeValues(
+    const nsTArray<Keyframe>& aKeyframes, dom::Element* aElement,
+    PseudoStyleType aPseudoType, const ComputedStyle* aComputedValues);
+
+static void BuildSegmentsFromValueEntries(
+    nsTArray<KeyframeValueEntry>& aEntries,
+    nsTArray<AnimationProperty>& aResult);
+
+static void GetKeyframeListFromPropertyIndexedKeyframe(
+    JSContext* aCx, dom::Document* aDocument, JS::Handle<JS::Value> aValue,
+    nsTArray<Keyframe>& aResult, ErrorResult& aRv);
+
+static void DistributeRange(const Range<Keyframe>& aRange);
+
+// ------------------------------------------------------------------
+//
+// Public API
+//
+// ------------------------------------------------------------------
+
+/* static */
+nsTArray<Keyframe> KeyframeUtils::GetKeyframesFromObject(
+    JSContext* aCx, dom::Document* aDocument, JS::Handle<JSObject*> aFrames,
+    const char* aContext, ErrorResult& aRv) {
+  MOZ_ASSERT(!aRv.Failed());
+
+  nsTArray<Keyframe> keyframes;
+
+  if (!aFrames) {
+    // The argument was explicitly null meaning no keyframes.
+    return keyframes;
+  }
+
+  // At this point we know we have an object. We try to convert it to a
+  // sequence of keyframes first, and if that fails due to not being iterable,
+  // we try to convert it to a property-indexed keyframe.
+  JS::Rooted<JS::Value> objectValue(aCx, JS::ObjectValue(*aFrames));
+  JS::ForOfIterator iter(aCx);
+  if (!iter.init(objectValue, JS::ForOfIterator::AllowNonIterable)) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return keyframes;
+  }
+
+  if (iter.valueIsIterable()) {
+    GetKeyframeListFromKeyframeSequence(aCx, aDocument, iter, keyframes,
+                                        aContext, aRv);
+  } else {
+    GetKeyframeListFromPropertyIndexedKeyframe(aCx, aDocument, objectValue,
+                                               keyframes, aRv);
+  }
+
+  if (aRv.Failed()) {
+    MOZ_ASSERT(keyframes.IsEmpty(),
+               "Should not set any keyframes when there is an error");
+    return keyframes;
+  }
+
+  return keyframes;
+}
+
+/* static */
+void KeyframeUtils::DistributeKeyframes(nsTArray<Keyframe>& aKeyframes) {
+  if (aKeyframes.IsEmpty()) {
+    return;
+  }
+
+  // If the first keyframe has an unspecified offset, fill it in with 0%.
+  // If there is only a single keyframe, then it gets 100%.
+  if (aKeyframes.Length() > 1) {
+    Keyframe& firstElement = aKeyframes[0];
+    firstElement.mComputedOffset = firstElement.mOffset.valueOr(0.0);
+    // We will fill in the last keyframe's offset below
+  } else {
+    Keyframe& lastElement = aKeyframes.LastElement();
+    lastElement.mComputedOffset = lastElement.mOffset.valueOr(1.0);
+  }
+
+  // Fill in remaining missing offsets.
+  const Keyframe* const last = &aKeyframes.LastElement();
+  const RangedPtr<Keyframe> begin(aKeyframes.Elements(), aKeyframes.Length());
+  RangedPtr<Keyframe> keyframeA = begin;
+  while (keyframeA != last) {
+    // Find keyframe A and keyframe B *between* which we will apply spacing.
+    RangedPtr<Keyframe> keyframeB = keyframeA + 1;
+    while (keyframeB->mOffset.isNothing() && keyframeB != last) {
+      ++keyframeB;
+    }
+    keyframeB->mComputedOffset = keyframeB->mOffset.valueOr(1.0);
+
+    // Fill computed offsets in (keyframe A, keyframe B).
+    DistributeRange(Range<Keyframe>(keyframeA, keyframeB + 1));
+    keyframeA = keyframeB;
+  }
+}
+
+/* static */
+nsTArray<AnimationProperty> KeyframeUtils::GetAnimationPropertiesFromKeyframes(
+    const nsTArray<Keyframe>& aKeyframes, dom::Element* aElement,
+    PseudoStyleType aPseudoType, const ComputedStyle* aStyle,
+    dom::CompositeOperation aEffectComposite) {
+  nsTArray<AnimationProperty> result;
+
+  const nsTArray<ComputedKeyframeValues> computedValues =
+      GetComputedKeyframeValues(aKeyframes, aElement, aPseudoType, aStyle);
+  if (computedValues.IsEmpty()) {
+    // In rare cases GetComputedKeyframeValues might fail and return an empty
+    // array, in which case we likewise return an empty array from here.
+    return result;
+  }
+
+  MOZ_ASSERT(aKeyframes.Length() == computedValues.Length(),
+             "Array length mismatch");
+
+  nsTArray<KeyframeValueEntry> entries(aKeyframes.Length());
+
+  const size_t len = aKeyframes.Length();
+  for (size_t i = 0; i < len; ++i) {
+    const Keyframe& frame = aKeyframes[i];
+    for (auto& value : computedValues[i]) {
+      MOZ_ASSERT(frame.mComputedOffset != Keyframe::kComputedOffsetNotSet,
+                 "Invalid computed offset");
+      KeyframeValueEntry* entry = entries.AppendElement();
+      entry->mOffset = frame.mComputedOffset;
+      entry->mProperty = value.mProperty;
+      entry->mValue = value.mValue;
+      entry->mTimingFunction = frame.mTimingFunction;
+      // The following assumes that CompositeOperation is a strict subset of
+      // CompositeOperationOrAuto.
+      entry->mComposite =
+          frame.mComposite == dom::CompositeOperationOrAuto::Auto
+              ? aEffectComposite
+              : static_cast<dom::CompositeOperation>(frame.mComposite);
+    }
+  }
+
+  BuildSegmentsFromValueEntries(entries, result);
+  return result;
+}
+
+/* static */
+bool KeyframeUtils::IsAnimatableProperty(const AnimatedPropertyID& aProperty) {
+  // Regardless of the backend type, treat the 'display' property as not
+  // animatable. (Servo will report it as being animatable, since it is
+  // in fact animatable by SMIL.)
+  if (aProperty.mID == eCSSProperty_display) {
+    return false;
+  }
+  return Servo_Property_IsAnimatable(&aProperty);
+}
+
+// ------------------------------------------------------------------
+//
+// Internal helpers
+//
+// ------------------------------------------------------------------
+
+/**
+ * Converts a JS object to an IDL sequence<Keyframe>.
+ *
+ * @param aCx The JSContext corresponding to |aIterator|.
+ * @param aDocument The document to use when parsing CSS properties.
+ * @param aIterator An already-initialized ForOfIterator for the JS
+ *   object to iterate over as a sequence.
+ * @param aResult The array into which the resulting Keyframe objects will be
+ *   appended.
+ * @param aContext The context string to prepend to thrown exceptions.
+ * @param aRv Out param to store any errors thrown by this function.
+ */
+static void GetKeyframeListFromKeyframeSequence(
+    JSContext* aCx, dom::Document* aDocument, JS::ForOfIterator& aIterator,
+    nsTArray<Keyframe>& aResult, const char* aContext, ErrorResult& aRv) {
+  MOZ_ASSERT(!aRv.Failed());
+  MOZ_ASSERT(aResult.IsEmpty());
+
+  // Convert the object in aIterator to a sequence of keyframes producing
+  // an array of Keyframe objects.
+  if (!ConvertKeyframeSequence(aCx, aDocument, aIterator, aContext, aResult)) {
+    aResult.Clear();
+    aRv.NoteJSContextException(aCx);
+    return;
+  }
+
+  // If the sequence<> had zero elements, we won't generate any
+  // keyframes.
+  if (aResult.IsEmpty()) {
+    return;
+  }
+
+  // Check that the keyframes are loosely sorted and with values all
+  // between 0% and 100%.
+  if (!HasValidOffsets(aResult)) {
+    aResult.Clear();
+    aRv.ThrowTypeError<dom::MSG_INVALID_KEYFRAME_OFFSETS>();
+    return;
+  }
+}
+
+/**
+ * Converts a JS object wrapped by the given JS::ForIfIterator to an
+ * IDL sequence<Keyframe> and stores the resulting Keyframe objects in
+ * aResult.
+ */
+static bool ConvertKeyframeSequence(JSContext* aCx, dom::Document* aDocument,
+                                    JS::ForOfIterator& aIterator,
+                                    const char* aContext,
+                                    nsTArray<Keyframe>& aResult) {
+  JS::Rooted<JS::Value> value(aCx);
+  // Parsing errors should only be reported after we have finished iterating
+  // through all values. If we have any early returns while iterating, we should
+  // ignore parsing errors.
+  IgnoredErrorResult parseEasingResult;
+
+  for (;;) {
+    bool done;
+    if (!aIterator.next(&value, &done)) {
+      return false;
+    }
+    if (done) {
+      break;
+    }
+    // Each value found when iterating the object must be an object
+    // or null/undefined (which gets treated as a default {} dictionary
+    // value).
+    if (!value.isObject() && !value.isNullOrUndefined()) {
+      dom::ThrowErrorMessage<dom::MSG_NOT_OBJECT>(
+          aCx, aContext, "Element of sequence<Keyframe> argument");
+      return false;
+    }
+
+    // Convert the JS value into a BaseKeyframe dictionary value.
+    dom::binding_detail::FastBaseKeyframe keyframeDict;
+    dom::BindingCallContext callCx(aCx, aContext);
+    if (!keyframeDict.Init(callCx, value,
+                           "Element of sequence<Keyframe> argument")) {
+      // This may happen if the value type of the member of BaseKeyframe is
+      // invalid. e.g. `offset` only accept a double value, so if we provide a
+      // string, we enter this branch.
+      // Besides, keyframeDict.Init() should throw a Type Error message already,
+      // so we don't have to do it again.
+      return false;
+    }
+
+    Keyframe* keyframe = aResult.AppendElement(fallible);
+    if (!keyframe) {
+      return false;
+    }
+
+    if (!keyframeDict.mOffset.IsNull()) {
+      keyframe->mOffset.emplace(keyframeDict.mOffset.Value());
+    }
+
+    if (StaticPrefs::dom_animations_api_compositing_enabled()) {
+      keyframe->mComposite = keyframeDict.mComposite;
+    }
+
+    // Look for additional property-values pairs on the object.
+    nsTArray<PropertyValuesPair> propertyValuePairs;
+    if (value.isObject()) {
+      JS::Rooted<JSObject*> object(aCx, &value.toObject());
+      if (!GetPropertyValuesPairs(aCx, object, ListAllowance::eDisallow,
+                                  propertyValuePairs)) {
+        return false;
+      }
+    }
+
+    if (!parseEasingResult.Failed()) {
+      keyframe->mTimingFunction =
+          TimingParams::ParseEasing(keyframeDict.mEasing, parseEasingResult);
+      // Even if the above fails, we still need to continue reading off all the
+      // properties since checking the validity of easing should be treated as
+      // a separate step that happens *after* all the other processing in this
+      // loop since (since it is never likely to be handled by WebIDL unlike the
+      // rest of this loop).
+    }
+
+    for (PropertyValuesPair& pair : propertyValuePairs) {
+      MOZ_ASSERT(pair.mValues.Length() == 1);
+
+      Maybe<PropertyValuePair> valuePair =
+          MakePropertyValuePair(pair.mProperty, pair.mValues[0], aDocument);
+      if (!valuePair) {
+        continue;
+      }
+      keyframe->mPropertyValues.AppendElement(std::move(valuePair.ref()));
+
+#ifdef DEBUG
+      // When we go to convert keyframes into arrays of property values we
+      // call StyleAnimation::ComputeValues. This should normally return true
+      // but in order to test the case where it does not, BaseKeyframeDict
+      // includes a chrome-only member that can be set to indicate that
+      // ComputeValues should fail for shorthand property values on that
+      // keyframe.
+      if (nsCSSProps::IsShorthand(pair.mProperty.mID) &&
+          keyframeDict.mSimulateComputeValuesFailure) {
+        MarkAsComputeValuesFailureKey(keyframe->mPropertyValues.LastElement());
+      }
+#endif
+    }
+  }
+
+  // Throw any errors we encountered while parsing 'easing' properties.
+  if (parseEasingResult.MaybeSetPendingException(aCx)) {
+    return false;
+  }
+
+  return true;
+}
+
+/**
+ * Reads the property-values pairs from the specified JS object.
+ *
+ * @param aObject The JS object to look at.
+ * @param aAllowLists If eAllow, values will be converted to
+ *   (DOMString or sequence<DOMString); if eDisallow, values
+ *   will be converted to DOMString.
+ * @param aResult The array into which the enumerated property-values
+ *   pairs will be stored.
+ * @return false on failure or JS exception thrown while interacting
+ *   with aObject; true otherwise.
+ */
+static bool GetPropertyValuesPairs(JSContext* aCx,
+                                   JS::Handle<JSObject*> aObject,
+                                   ListAllowance aAllowLists,
+                                   nsTArray<PropertyValuesPair>& aResult) {
+  nsTArray<AdditionalProperty> properties;
+
+  // Iterate over all the properties on aObject and append an
+  // entry to properties for them.
+  //
+  // We don't compare the jsids that we encounter with those for
+  // the explicit dictionary members, since we know that none
+  // of the CSS property IDL names clash with them.
+  JS::Rooted<JS::IdVector> ids(aCx, JS::IdVector(aCx));
+  if (!JS_Enumerate(aCx, aObject, &ids)) {
+    return false;
+  }
+  for (size_t i = 0, n = ids.length(); i < n; i++) {
+    nsAutoJSCString propName;
+    if (!propName.init(aCx, ids[i])) {
+      return false;
+    }
+
+    // Basically, we have to handle "cssOffset" and "cssFloat" specially here:
+    // "cssOffset" => eCSSProperty_offset
+    // "cssFloat"  => eCSSProperty_float
+    // This means if the attribute is the string "cssOffset"/"cssFloat", we use
+    // CSS "offset"/"float" property.
+    // https://drafts.csswg.org/web-animations/#property-name-conversion
+    nsCSSPropertyID propertyID = nsCSSPropertyID::eCSSProperty_UNKNOWN;
+    if (nsCSSProps::IsCustomPropertyName(propName)) {
+      propertyID = eCSSPropertyExtra_variable;
+    } else if (propName.EqualsLiteral("cssOffset")) {
+      propertyID = nsCSSPropertyID::eCSSProperty_offset;
+    } else if (propName.EqualsLiteral("cssFloat")) {
+      propertyID = nsCSSPropertyID::eCSSProperty_float;
+    } else if (!propName.EqualsLiteral("offset") &&
+               !propName.EqualsLiteral("float")) {
+      propertyID = nsCSSProps::LookupPropertyByIDLName(
+          propName, CSSEnabledState::ForAllContent);
+    }
+
+    // TODO(zrhoffman, bug 1811897) Add test coverage for removing the `--`
+    // prefix here.
+    AnimatedPropertyID property =
+        propertyID == eCSSPropertyExtra_variable
+            ? AnimatedPropertyID(
+                  NS_Atomize(Substring(propName, 2, propName.Length() - 2)))
+            : AnimatedPropertyID(propertyID);
+
+    if (KeyframeUtils::IsAnimatableProperty(property)) {
+      properties.AppendElement(AdditionalProperty{std::move(property), i});
+    }
+  }
+
+  // Sort the entries by IDL name and then get each value and
+  // convert it either to a DOMString or to a
+  // (DOMString or sequence<DOMString>), depending on aAllowLists,
+  // and build up aResult.
+  properties.Sort(AdditionalProperty::PropertyComparator());
+
+  for (AdditionalProperty& p : properties) {
+    JS::Rooted<JS::Value> value(aCx);
+    if (!JS_GetPropertyById(aCx, aObject, ids[p.mJsidIndex], &value)) {
+      return false;
+    }
+    PropertyValuesPair* pair = aResult.AppendElement();
+    pair->mProperty = p.mProperty;
+    if (!AppendStringOrStringSequenceToArray(aCx, value, aAllowLists,
+                                             pair->mValues)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+/**
+ * Converts aValue to DOMString, if aAllowLists is eDisallow, or
+ * to (DOMString or sequence<DOMString>) if aAllowLists is aAllow.
+ * The resulting strings are appended to aValues.
+ */
+static bool AppendStringOrStringSequenceToArray(JSContext* aCx,
+                                                JS::Handle<JS::Value> aValue,
+                                                ListAllowance aAllowLists,
+                                                nsTArray<nsCString>& aValues) {
+  if (aAllowLists == ListAllowance::eAllow && aValue.isObject()) {
+    // The value is an object, and we want to allow lists; convert
+    // aValue to (DOMString or sequence<DOMString>).
+    JS::ForOfIterator iter(aCx);
+    if (!iter.init(aValue, JS::ForOfIterator::AllowNonIterable)) {
+      return false;
+    }
+    if (iter.valueIsIterable()) {
+      // If the object is iterable, convert it to sequence<DOMString>.
+      JS::Rooted<JS::Value> element(aCx);
+      for (;;) {
+        bool done;
+        if (!iter.next(&element, &done)) {
+          return false;
+        }
+        if (done) {
+          break;
+        }
+        if (!AppendValueAsString(aCx, aValues, element)) {
+          return false;
+        }
+      }
+      return true;
+    }
+  }
+
+  // Either the object is not iterable, or aAllowLists doesn't want
+  // a list; convert it to DOMString.
+  if (!AppendValueAsString(aCx, aValues, aValue)) {
+    return false;
+  }
+
+  return true;
+}
+
+/**
+ * Converts aValue to DOMString and appends it to aValues.
+ */
+static bool AppendValueAsString(JSContext* aCx, nsTArray<nsCString>& aValues,
+                                JS::Handle<JS::Value> aValue) {
+  return ConvertJSValueToString(aCx, aValue, dom::eStringify, dom::eStringify,
+                                *aValues.AppendElement());
+}
+
+static void ReportInvalidPropertyValueToConsole(
+    const AnimatedPropertyID& aProperty,
+    const nsACString& aInvalidPropertyValue, dom::Document* aDoc) {
+  AutoTArray<nsString, 2> params;
+  params.AppendElement(NS_ConvertUTF8toUTF16(aInvalidPropertyValue));
+  aProperty.ToString(*params.AppendElement());
+  nsContentUtils::ReportToConsole(nsIScriptError::warningFlag, "Animation"_ns,
+                                  aDoc, nsContentUtils::eDOM_PROPERTIES,
+                                  "InvalidKeyframePropertyValue", params);
+}
+
+/**
+ * Construct a PropertyValuePair parsing the given string into a suitable
+ * nsCSSValue object.
+ *
+ * @param aProperty The CSS property.
+ * @param aStringValue The property value to parse.
+ * @param aDocument The document to use when parsing.
+ * @return The constructed PropertyValuePair, or Nothing() if |aStringValue| is
+ *   an invalid property value.
+ */
+static Maybe<PropertyValuePair> MakePropertyValuePair(
+    const AnimatedPropertyID& aProperty, const nsACString& aStringValue,
+    dom::Document* aDocument) {
+  MOZ_ASSERT(aDocument);
+  Maybe<PropertyValuePair> result;
+
+  ServoCSSParser::ParsingEnvironment env =
+      ServoCSSParser::GetParsingEnvironment(aDocument);
+  RefPtr<StyleLockedDeclarationBlock> servoDeclarationBlock =
+      ServoCSSParser::ParseProperty(aProperty, aStringValue, env,
+                                    StyleParsingMode::DEFAULT);
+
+  if (servoDeclarationBlock) {
+    result.emplace(aProperty, std::move(servoDeclarationBlock));
+  } else {
+    ReportInvalidPropertyValueToConsole(aProperty, aStringValue, aDocument);
+  }
+  return result;
+}
+
+/**
+ * Checks that the given keyframes are loosely ordered (each keyframe's
+ * offset that is not null is greater than or equal to the previous
+ * non-null offset) and that all values are within the range [0.0, 1.0].
+ *
+ * @return true if the keyframes' offsets are correctly ordered and
+ *   within range; false otherwise.
+ */
+static bool HasValidOffsets(const nsTArray<Keyframe>& aKeyframes) {
+  double offset = 0.0;
+  for (const Keyframe& keyframe : aKeyframes) {
+    if (keyframe.mOffset) {
+      double thisOffset = keyframe.mOffset.value();
+      if (thisOffset < offset || thisOffset > 1.0f) {
+        return false;
+      }
+      offset = thisOffset;
+    }
+  }
+  return true;
+}
+
+#ifdef DEBUG
+/**
+ * Takes a property-value pair for a shorthand property and modifies the
+ * value to indicate that when we call StyleAnimationValue::ComputeValues on
+ * that value we should behave as if that function had failed.
+ *
+ * @param aPair The PropertyValuePair to modify. |aPair.mProperty| must be
+ *              a shorthand property.
+ */
+static void MarkAsComputeValuesFailureKey(PropertyValuePair& aPair) {
+  MOZ_ASSERT(nsCSSProps::IsShorthand(aPair.mProperty.mID),
+             "Only shorthand property values can be marked as failure values");
+
+  aPair.mSimulateComputeValuesFailure = true;
+}
+
+#endif
+
+/**
+ * The variation of the above function. This is for Servo backend.
+ */
+static nsTArray<ComputedKeyframeValues> GetComputedKeyframeValues(
+    const nsTArray<Keyframe>& aKeyframes, dom::Element* aElement,
+    PseudoStyleType aPseudoType, const ComputedStyle* aComputedStyle) {
+  MOZ_ASSERT(aElement);
+
+  nsTArray<ComputedKeyframeValues> result;
+
+  nsPresContext* presContext = nsContentUtils::GetContextForContent(aElement);
+  if (!presContext) {
+    // This has been reported to happen with some combinations of content
+    // (particularly involving resize events and layout flushes? See bug 1407898
+    // and bug 1408420) but no reproducible steps have been found.
+    // For now we just return an empty array.
+    return result;
+  }
+
+  result = presContext->StyleSet()->GetComputedKeyframeValuesFor(
+      aKeyframes, aElement, aPseudoType, aComputedStyle);
+  return result;
+}
+
+static void AppendInitialSegment(AnimationProperty* aAnimationProperty,
+                                 const KeyframeValueEntry& aFirstEntry) {
+  AnimationPropertySegment* segment =
+      aAnimationProperty->mSegments.AppendElement();
+  segment->mFromKey = 0.0f;
+  segment->mToKey = aFirstEntry.mOffset;
+  segment->mToValue = aFirstEntry.mValue;
+  segment->mToComposite = aFirstEntry.mComposite;
+}
+
+static void AppendFinalSegment(AnimationProperty* aAnimationProperty,
+                               const KeyframeValueEntry& aLastEntry) {
+  AnimationPropertySegment* segment =
+      aAnimationProperty->mSegments.AppendElement();
+  segment->mFromKey = aLastEntry.mOffset;
+  segment->mFromValue = aLastEntry.mValue;
+  segment->mFromComposite = aLastEntry.mComposite;
+  segment->mToKey = 1.0f;
+  segment->mTimingFunction = aLastEntry.mTimingFunction;
+}
+
+// Returns a newly created AnimationProperty if one was created to fill-in the
+// missing keyframe, nullptr otherwise (if we decided not to fill the keyframe
+// becase we don't support implicit keyframes).
+static AnimationProperty* HandleMissingInitialKeyframe(
+    nsTArray<AnimationProperty>& aResult, const KeyframeValueEntry& aEntry) {
+  MOZ_ASSERT(aEntry.mOffset != 0.0f,
+             "The offset of the entry should not be 0.0");
+
+  AnimationProperty* result = aResult.AppendElement();
+  result->mProperty = aEntry.mProperty;
+
+  AppendInitialSegment(result, aEntry);
+
+  return result;
+}
+
+static void HandleMissingFinalKeyframe(
+    nsTArray<AnimationProperty>& aResult, const KeyframeValueEntry& aEntry,
+    AnimationProperty* aCurrentAnimationProperty) {
+  MOZ_ASSERT(aEntry.mOffset != 1.0f,
+             "The offset of the entry should not be 1.0");
+
+  // If |aCurrentAnimationProperty| is nullptr, that means this is the first
+  // entry for the property, we have to append a new AnimationProperty for this
+  // property.
+  if (!aCurrentAnimationProperty) {
+    aCurrentAnimationProperty = aResult.AppendElement();
+    aCurrentAnimationProperty->mProperty = aEntry.mProperty;
+
+    // If we have only one entry whose offset is neither 1 nor 0 for this
+    // property, we need to append the initial segment as well.
+    if (aEntry.mOffset != 0.0f) {
+      AppendInitialSegment(aCurrentAnimationProperty, aEntry);
+    }
+  }
+  AppendFinalSegment(aCurrentAnimationProperty, aEntry);
+}
+
+/**
+ * Builds an array of AnimationProperty objects to represent the keyframe
+ * animation segments in aEntries.
+ */
+static void BuildSegmentsFromValueEntries(
+    nsTArray<KeyframeValueEntry>& aEntries,
+    nsTArray<AnimationProperty>& aResult) {
+  if (aEntries.IsEmpty()) {
+    return;
+  }
+
+  // Sort the KeyframeValueEntry objects so that all entries for a given
+  // property are together, and the entries are sorted by offset otherwise.
+  std::stable_sort(aEntries.begin(), aEntries.end(),
+                   &KeyframeValueEntry::PropertyOffsetComparator::LessThan);
+
+  // For a given index i, we want to generate a segment from aEntries[i]
+  // to aEntries[j], if:
+  //
+  //   * j > i,
+  //   * aEntries[i + 1]'s offset/property is different from aEntries[i]'s, and
+  //   * aEntries[j - 1]'s offset/property is different from aEntries[j]'s.
+  //
+  // That will eliminate runs of same offset/property values where there's no
+  // point generating zero length segments in the middle of the animation.
+  //
+  // Additionally we need to generate a zero length segment at offset 0 and at
+  // offset 1, if we have multiple values for a given property at that offset,
+  // since we need to retain the very first and very last value so they can
+  // be used for reverse and forward filling.
+  //
+  // Typically, for each property in |aEntries|, we expect there to be at least
+  // one KeyframeValueEntry with offset 0.0, and at least one with offset 1.0.
+  // However, since it is possible that when building |aEntries|, the call to
+  // StyleAnimationValue::ComputeValues might fail, this can't be guaranteed.
+  // Furthermore, if additive animation is disabled, the following loop takes
+  // care to identify properties that lack a value at offset 0.0/1.0 and drops
+  // those properties from |aResult|.
+
+  AnimatedPropertyID lastProperty(eCSSProperty_UNKNOWN);
+  AnimationProperty* animationProperty = nullptr;
+
+  size_t i = 0, n = aEntries.Length();
+
+  while (i < n) {
+    // If we've reached the end of the array of entries, synthesize a final (and
+    // initial) segment if necessary.
+    if (i + 1 == n) {
+      if (aEntries[i].mOffset != 1.0f) {
+        HandleMissingFinalKeyframe(aResult, aEntries[i], animationProperty);
+      } else if (aEntries[i].mOffset == 1.0f && !animationProperty) {
+        // If the last entry with offset 1 and no animation property, that means
+        // it is the only entry for this property so append a single segment
+        // from 0 offset to |aEntry[i].offset|.
+        Unused << HandleMissingInitialKeyframe(aResult, aEntries[i]);
+      }
+      animationProperty = nullptr;
+      break;
+    }
+
+    MOZ_ASSERT(
+        aEntries[i].mProperty.IsValid() && aEntries[i + 1].mProperty.IsValid(),
+        "Each entry should specify a valid property");
+
+    // No keyframe for this property at offset 0.
+    if (aEntries[i].mProperty != lastProperty && aEntries[i].mOffset != 0.0f) {
+      // If we don't support additive animation we can't fill in the missing
+      // keyframes and we should just skip this property altogether. Since the
+      // entries are sorted by offset for a given property, and since we don't
+      // update |lastProperty|, we will keep hitting this condition until we
+      // change property.
+      animationProperty = HandleMissingInitialKeyframe(aResult, aEntries[i]);
+      if (animationProperty) {
+        lastProperty = aEntries[i].mProperty;
+      } else {
+        // Skip this entry if we did not handle the missing entry.
+        ++i;
+        continue;
+      }
+    }
+
+    // Skip this entry if the next entry has the same offset except for initial
+    // and final ones. We will handle missing keyframe in the next loop
+    // if the property is changed on the next entry.
+    if (aEntries[i].mProperty == aEntries[i + 1].mProperty &&
+        aEntries[i].mOffset == aEntries[i + 1].mOffset &&
+        aEntries[i].mOffset != 1.0f && aEntries[i].mOffset != 0.0f) {
+      ++i;
+      continue;
+    }
+
+    // No keyframe for this property at offset 1.
+    if (aEntries[i].mProperty != aEntries[i + 1].mProperty &&
+        aEntries[i].mOffset != 1.0f) {
+      HandleMissingFinalKeyframe(aResult, aEntries[i], animationProperty);
+      // Move on to new property.
+      animationProperty = nullptr;
+      ++i;
+      continue;
+    }
+
+    // Starting from i + 1, determine the next [i, j] interval from which to
+    // generate a segment. Basically, j is i + 1, but there are some special
+    // cases for offset 0 and 1, so we need to handle them specifically.
+    // Note: From this moment, we make sure [i + 1] is valid and
+    //       there must be an initial entry (i.e. mOffset = 0.0) and
+    //       a final entry (i.e. mOffset = 1.0). Besides, all the entries
+    //       with the same offsets except for initial/final ones are filtered
+    //       out already.
+    size_t j = i + 1;
+    if (aEntries[i].mOffset == 0.0f && aEntries[i + 1].mOffset == 0.0f) {
+      // We need to generate an initial zero-length segment.
+      MOZ_ASSERT(aEntries[i].mProperty == aEntries[i + 1].mProperty);
+      while (j + 1 < n && aEntries[j + 1].mOffset == 0.0f &&
+             aEntries[j + 1].mProperty == aEntries[j].mProperty) {
+        ++j;
+      }
+    } else if (aEntries[i].mOffset == 1.0f) {
+      if (aEntries[i + 1].mOffset == 1.0f &&
+          aEntries[i + 1].mProperty == aEntries[i].mProperty) {
+        // We need to generate a final zero-length segment.
+        while (j + 1 < n && aEntries[j + 1].mOffset == 1.0f &&
+               aEntries[j + 1].mProperty == aEntries[j].mProperty) {
+          ++j;
+        }
+      } else {
+        // New property.
+        MOZ_ASSERT(aEntries[i].mProperty != aEntries[i + 1].mProperty);
+        animationProperty = nullptr;
+        ++i;
+        continue;
+      }
+    }
+
+    // If we've moved on to a new property, create a new AnimationProperty
+    // to insert segments into.
+    if (aEntries[i].mProperty != lastProperty) {
+      MOZ_ASSERT(aEntries[i].mOffset == 0.0f);
+      MOZ_ASSERT(!animationProperty);
+      animationProperty = aResult.AppendElement();
+      animationProperty->mProperty = aEntries[i].mProperty;
+      lastProperty = aEntries[i].mProperty;
+    }
+
+    MOZ_ASSERT(animationProperty, "animationProperty should be valid pointer.");
+
+    // Now generate the segment.
+    AnimationPropertySegment* segment =
+        animationProperty->mSegments.AppendElement();
+    segment->mFromKey = aEntries[i].mOffset;
+    segment->mToKey = aEntries[j].mOffset;
+    segment->mFromValue = aEntries[i].mValue;
+    segment->mToValue = aEntries[j].mValue;
+    segment->mTimingFunction = aEntries[i].mTimingFunction;
+    segment->mFromComposite = aEntries[i].mComposite;
+    segment->mToComposite = aEntries[j].mComposite;
+
+    i = j;
+  }
+}
+
+/**
+ * Converts a JS object representing a property-indexed keyframe into
+ * an array of Keyframe objects.
+ *
+ * @param aCx The JSContext for |aValue|.
+ * @param aDocument The document to use when parsing CSS properties.
+ * @param aValue The JS object.
+ * @param aResult The array into which the resulting AnimationProperty
+ *   objects will be appended.
+ * @param aRv Out param to store any errors thrown by this function.
+ */
+static void GetKeyframeListFromPropertyIndexedKeyframe(
+    JSContext* aCx, dom::Document* aDocument, JS::Handle<JS::Value> aValue,
+    nsTArray<Keyframe>& aResult, ErrorResult& aRv) {
+  MOZ_ASSERT(aValue.isObject());
+  MOZ_ASSERT(aResult.IsEmpty());
+  MOZ_ASSERT(!aRv.Failed());
+
+  // Convert the object to a property-indexed keyframe dictionary to
+  // get its explicit dictionary members.
+  dom::binding_detail::FastBasePropertyIndexedKeyframe keyframeDict;
+  // XXXbz Pass in the method name from callers and set up a BindingCallContext?
+  if (!keyframeDict.Init(aCx, aValue, "BasePropertyIndexedKeyframe argument")) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return;
+  }
+
+  // Get all the property--value-list pairs off the object.
+  JS::Rooted<JSObject*> object(aCx, &aValue.toObject());
+  nsTArray<PropertyValuesPair> propertyValuesPairs;
+  if (!GetPropertyValuesPairs(aCx, object, ListAllowance::eAllow,
+                              propertyValuesPairs)) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return;
+  }
+
+  // Create a set of keyframes for each property.
+  nsTHashMap<nsFloatHashKey, Keyframe> processedKeyframes;
+  for (const PropertyValuesPair& pair : propertyValuesPairs) {
+    size_t count = pair.mValues.Length();
+    if (count == 0) {
+      // No animation values for this property.
+      continue;
+    }
+
+    size_t n = pair.mValues.Length() - 1;
+    size_t i = 0;
+
+    for (const nsCString& stringValue : pair.mValues) {
+      // For single-valued lists, the single value should be added to a
+      // keyframe with offset 1.
+      double offset = n ? i++ / double(n) : 1;
+      Keyframe& keyframe = processedKeyframes.LookupOrInsert(offset);
+      if (keyframe.mPropertyValues.IsEmpty()) {
+        keyframe.mComputedOffset = offset;
+      }
+
+      Maybe<PropertyValuePair> valuePair =
+          MakePropertyValuePair(pair.mProperty, stringValue, aDocument);
+      if (!valuePair) {
+        continue;
+      }
+      keyframe.mPropertyValues.AppendElement(std::move(valuePair.ref()));
+    }
+  }
+
+  aResult.SetCapacity(processedKeyframes.Count());
+  std::transform(processedKeyframes.begin(), processedKeyframes.end(),
+                 MakeBackInserter(aResult), [](auto& entry) {
+                   return std::move(*entry.GetModifiableData());
+                 });
+
+  aResult.Sort(ComputedOffsetComparator());
+
+  // Fill in any specified offsets
+  //
+  // This corresponds to step 5, "Otherwise," branch, substeps 5-6 of
+  // https://drafts.csswg.org/web-animations/#processing-a-keyframes-argument
+  const FallibleTArray<Nullable<double>>* offsets = nullptr;
+  AutoTArray<Nullable<double>, 1> singleOffset;
+  auto& offset = keyframeDict.mOffset;
+  if (offset.IsDouble()) {
+    singleOffset.AppendElement(offset.GetAsDouble());
+    // dom::Sequence is a fallible but AutoTArray is infallible and we need to
+    // point to one or the other. Fortunately, fallible and infallible array
+    // types can be implicitly converted provided they are const.
+    const FallibleTArray<Nullable<double>>& asFallibleArray = singleOffset;
+    offsets = &asFallibleArray;
+  } else if (offset.IsDoubleOrNullSequence()) {
+    offsets = &offset.GetAsDoubleOrNullSequence();
+  }
+  // If offset.IsNull() is true, then we want to leave the mOffset member of
+  // each keyframe with its initialized value of null. By leaving |offsets|
+  // as nullptr here, we skip updating mOffset below.
+
+  size_t offsetsToFill =
+      offsets ? std::min(offsets->Length(), aResult.Length()) : 0;
+  for (size_t i = 0; i < offsetsToFill; i++) {
+    if (!offsets->ElementAt(i).IsNull()) {
+      aResult[i].mOffset.emplace(offsets->ElementAt(i).Value());
+    }
+  }
+
+  // Check that the keyframes are loosely sorted and that any specified offsets
+  // are between 0.0 and 1.0 inclusive.
+  //
+  // This corresponds to steps 6-7 of
+  // https://drafts.csswg.org/web-animations/#processing-a-keyframes-argument
+  //
+  // In the spec, TypeErrors arising from invalid offsets and easings are thrown
+  // at the end of the procedure since it assumes we initially store easing
+  // values as strings and then later parse them.
+  //
+  // However, we will parse easing members immediately when we process them
+  // below. In order to maintain the relative order in which TypeErrors are
+  // thrown according to the spec, namely exceptions arising from invalid
+  // offsets are thrown before exceptions arising from invalid easings, we check
+  // the offsets here.
+  if (!HasValidOffsets(aResult)) {
+    aResult.Clear();
+    aRv.ThrowTypeError<dom::MSG_INVALID_KEYFRAME_OFFSETS>();
+    return;
+  }
+
+  // Fill in any easings.
+  //
+  // This corresponds to step 5, "Otherwise," branch, substeps 7-11 of
+  // https://drafts.csswg.org/web-animations/#processing-a-keyframes-argument
+  FallibleTArray<Maybe<StyleComputedTimingFunction>> easings;
+  auto parseAndAppendEasing = [&](const nsACString& easingString,
+                                  ErrorResult& aRv) {
+    auto easing = TimingParams::ParseEasing(easingString, aRv);
+    if (!aRv.Failed() && !easings.AppendElement(std::move(easing), fallible)) {
+      aRv.Throw(NS_ERROR_OUT_OF_MEMORY);
+    }
+  };
+
+  auto& easing = keyframeDict.mEasing;
+  if (easing.IsUTF8String()) {
+    parseAndAppendEasing(easing.GetAsUTF8String(), aRv);
+    if (aRv.Failed()) {
+      aResult.Clear();
+      return;
+    }
+  } else {
+    for (const auto& easingString : easing.GetAsUTF8StringSequence()) {
+      parseAndAppendEasing(easingString, aRv);
+      if (aRv.Failed()) {
+        aResult.Clear();
+        return;
+      }
+    }
+  }
+
+  // If |easings| is empty, then we are supposed to fill it in with the value
+  // "linear" and then repeat the list as necessary.
+  //
+  // However, for Keyframe.mTimingFunction we represent "linear" as a None
+  // value. Since we have not assigned 'mTimingFunction' for any of the
+  // keyframes in |aResult| they will already have their initial None value
+  // (i.e. linear). As a result, if |easings| is empty, we don't need to do
+  // anything.
+  if (!easings.IsEmpty()) {
+    for (size_t i = 0; i < aResult.Length(); i++) {
+      aResult[i].mTimingFunction = easings[i % easings.Length()];
+    }
+  }
+
+  // Fill in any composite operations.
+  //
+  // This corresponds to step 5, "Otherwise," branch, substep 12 of
+  // https://drafts.csswg.org/web-animations/#processing-a-keyframes-argument
+  if (StaticPrefs::dom_animations_api_compositing_enabled()) {
+    const FallibleTArray<dom::CompositeOperationOrAuto>* compositeOps = nullptr;
+    AutoTArray<dom::CompositeOperationOrAuto, 1> singleCompositeOp;
+    auto& composite = keyframeDict.mComposite;
+    if (composite.IsCompositeOperationOrAuto()) {
+      singleCompositeOp.AppendElement(
+          composite.GetAsCompositeOperationOrAuto());
+      const FallibleTArray<dom::CompositeOperationOrAuto>& asFallibleArray =
+          singleCompositeOp;
+      compositeOps = &asFallibleArray;
+    } else if (composite.IsCompositeOperationOrAutoSequence()) {
+      compositeOps = &composite.GetAsCompositeOperationOrAutoSequence();
+    }
+
+    // Fill in and repeat as needed.
+    if (compositeOps && !compositeOps->IsEmpty()) {
+      size_t length = compositeOps->Length();
+      for (size_t i = 0; i < aResult.Length(); i++) {
+        aResult[i].mComposite = compositeOps->ElementAt(i % length);
+      }
+    }
+  }
+}
+
+/**
+ * Distribute the offsets of all keyframes in between the endpoints of the
+ * given range.
+ *
+ * @param aRange The sequence of keyframes between whose endpoints we should
+ * distribute offsets.
+ */
+static void DistributeRange(const Range<Keyframe>& aRange) {
+  const Range<Keyframe> rangeToAdjust =
+      Range<Keyframe>(aRange.begin() + 1, aRange.end() - 1);
+  const size_t n = aRange.length() - 1;
+  const double startOffset = aRange[0].mComputedOffset;
+  const double diffOffset = aRange[n].mComputedOffset - startOffset;
+  for (auto iter = rangeToAdjust.begin(); iter != rangeToAdjust.end(); ++iter) {
+    size_t index = iter - aRange.begin();
+    iter->mComputedOffset = startOffset + double(index) / n * diffOffset;
+  }
+}
+
+}  // namespace mozilla