Add m-esr52 at 52.6.0

1 files changed, 1667 insertions, 0 deletions

diff --git a/dom/animation/KeyframeUtils.cpp b/dom/animation/KeyframeUtils.cpp
new file mode 100644
index 000000000..8e396f84c
--- /dev/null
+++ b/dom/animation/KeyframeUtils.cpp
@@ -0,0 +1,1667 @@
+/* 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 "mozilla/AnimationUtils.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/Move.h"
+#include "mozilla/RangedArray.h"
+#include "mozilla/ServoBindings.h"
+#include "mozilla/StyleAnimationValue.h"
+#include "mozilla/TimingParams.h"
+#include "mozilla/dom/BaseKeyframeTypesBinding.h" // For FastBaseKeyframe etc.
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/KeyframeEffectBinding.h"
+#include "mozilla/dom/KeyframeEffectReadOnly.h" // For PropertyValuesPair etc.
+#include "jsapi.h" // For ForOfIterator etc.
+#include "nsClassHashtable.h"
+#include "nsCSSParser.h"
+#include "nsCSSPropertyIDSet.h"
+#include "nsCSSProps.h"
+#include "nsCSSPseudoElements.h" // For CSSPseudoElementType
+#include "nsTArray.h"
+#include <algorithm> // For std::stable_sort
+
+namespace mozilla {
+
+// ------------------------------------------------------------------
+//
+// Internal data types
+//
+// ------------------------------------------------------------------
+
+// This is used while calculating paced spacing. If the keyframe is not pacable,
+// we set its cumulative distance to kNotPaceable, so we can use this to check.
+const double kNotPaceable = -1.0;
+
+// For the aAllowList parameter of AppendStringOrStringSequence and
+// GetPropertyValuesPairs.
+enum class ListAllowance { eDisallow, eAllow };
+
+/**
+ * A comparator to sort nsCSSPropertyID values such that longhands are sorted
+ * before shorthands, and shorthands with fewer components are sorted before
+ * shorthands with more components.
+ *
+ * Using this allows us to prioritize values specified by longhands (or smaller
+ * shorthand subsets) when longhands and shorthands are both specified
+ * on the one keyframe.
+ *
+ * Example orderings that result from this:
+ *
+ *   margin-left, margin
+ *
+ * and:
+ *
+ *   border-top-color, border-color, border-top, border
+ */
+class PropertyPriorityComparator
+{
+public:
+  PropertyPriorityComparator()
+    : mSubpropertyCountInitialized(false) {}
+
+  bool Equals(nsCSSPropertyID aLhs, nsCSSPropertyID aRhs) const
+  {
+    return aLhs == aRhs;
+  }
+
+  bool LessThan(nsCSSPropertyID aLhs,
+                nsCSSPropertyID aRhs) const
+  {
+    bool isShorthandLhs = nsCSSProps::IsShorthand(aLhs);
+    bool isShorthandRhs = nsCSSProps::IsShorthand(aRhs);
+
+    if (isShorthandLhs) {
+      if (isShorthandRhs) {
+        // First, sort shorthands by the number of longhands they have.
+        uint32_t subpropCountLhs = SubpropertyCount(aLhs);
+        uint32_t subpropCountRhs = SubpropertyCount(aRhs);
+        if (subpropCountLhs != subpropCountRhs) {
+          return subpropCountLhs < subpropCountRhs;
+        }
+        // Otherwise, sort by IDL name below.
+      } else {
+        // Put longhands before shorthands.
+        return false;
+      }
+    } else {
+      if (isShorthandRhs) {
+        // Put longhands before shorthands.
+        return true;
+      }
+    }
+    // For two longhand properties, or two shorthand with the same number
+    // of longhand components, sort by IDL name.
+    return nsCSSProps::PropertyIDLNameSortPosition(aLhs) <
+           nsCSSProps::PropertyIDLNameSortPosition(aRhs);
+  }
+
+  uint32_t SubpropertyCount(nsCSSPropertyID aProperty) const
+  {
+    if (!mSubpropertyCountInitialized) {
+      PodZero(&mSubpropertyCount);
+      mSubpropertyCountInitialized = true;
+    }
+    if (mSubpropertyCount[aProperty] == 0) {
+      uint32_t count = 0;
+      CSSPROPS_FOR_SHORTHAND_SUBPROPERTIES(
+          p, aProperty, CSSEnabledState::eForAllContent) {
+        ++count;
+      }
+      mSubpropertyCount[aProperty] = count;
+    }
+    return mSubpropertyCount[aProperty];
+  }
+
+private:
+  // Cache of shorthand subproperty counts.
+  mutable RangedArray<
+    uint32_t,
+    eCSSProperty_COUNT_no_shorthands,
+    eCSSProperty_COUNT - eCSSProperty_COUNT_no_shorthands> mSubpropertyCount;
+  mutable bool mSubpropertyCountInitialized;
+};
+
+/**
+ * Adaptor for PropertyPriorityComparator to sort objects which have
+ * a mProperty member.
+ */
+template <typename T>
+class TPropertyPriorityComparator : PropertyPriorityComparator
+{
+public:
+  bool Equals(const T& aLhs, const T& aRhs) const
+  {
+    return PropertyPriorityComparator::Equals(aLhs.mProperty, aRhs.mProperty);
+  }
+  bool LessThan(const T& aLhs, const T& aRhs) const
+  {
+    return PropertyPriorityComparator::LessThan(aLhs.mProperty, aRhs.mProperty);
+  }
+};
+
+/**
+ * Iterator to walk through a PropertyValuePair array using the ordering
+ * provided by PropertyPriorityComparator.
+ */
+class PropertyPriorityIterator
+{
+public:
+  explicit PropertyPriorityIterator(
+    const nsTArray<PropertyValuePair>& aProperties)
+    : mProperties(aProperties)
+  {
+    mSortedPropertyIndices.SetCapacity(mProperties.Length());
+    for (size_t i = 0, len = mProperties.Length(); i < len; ++i) {
+      PropertyAndIndex propertyIndex = { mProperties[i].mProperty, i };
+      mSortedPropertyIndices.AppendElement(propertyIndex);
+    }
+    mSortedPropertyIndices.Sort(PropertyAndIndex::Comparator());
+  }
+
+  class Iter
+  {
+  public:
+    explicit Iter(const PropertyPriorityIterator& aParent)
+      : mParent(aParent)
+      , mIndex(0) { }
+
+    static Iter EndIter(const PropertyPriorityIterator &aParent)
+    {
+      Iter iter(aParent);
+      iter.mIndex = aParent.mSortedPropertyIndices.Length();
+      return iter;
+    }
+
+    bool operator!=(const Iter& aOther) const
+    {
+      return mIndex != aOther.mIndex;
+    }
+
+    Iter& operator++()
+    {
+      MOZ_ASSERT(mIndex + 1 <= mParent.mSortedPropertyIndices.Length(),
+                 "Should not seek past end iterator");
+      mIndex++;
+      return *this;
+    }
+
+    const PropertyValuePair& operator*()
+    {
+      MOZ_ASSERT(mIndex < mParent.mSortedPropertyIndices.Length(),
+                 "Should not try to dereference an end iterator");
+      return mParent.mProperties[mParent.mSortedPropertyIndices[mIndex].mIndex];
+    }
+
+  private:
+    const PropertyPriorityIterator& mParent;
+    size_t mIndex;
+  };
+
+  Iter begin() { return Iter(*this); }
+  Iter end()   { return Iter::EndIter(*this); }
+
+private:
+  struct PropertyAndIndex
+  {
+    nsCSSPropertyID mProperty;
+    size_t mIndex; // Index of mProperty within mProperties
+
+    typedef TPropertyPriorityComparator<PropertyAndIndex> Comparator;
+  };
+
+  const nsTArray<PropertyValuePair>& mProperties;
+  nsTArray<PropertyAndIndex> mSortedPropertyIndices;
+};
+
+/**
+ * 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
+{
+  nsCSSPropertyID mProperty;
+  nsTArray<nsString> mValues;
+
+  typedef TPropertyPriorityComparator<PropertyValuesPair> Comparator;
+};
+
+/**
+ * An additional property (for a property-values pair) found on a
+ * BaseKeyframe or BasePropertyIndexedKeyframe object.
+ */
+struct AdditionalProperty
+{
+  nsCSSPropertyID mProperty;
+  size_t mJsidIndex;        // 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
+    {
+      return nsCSSProps::PropertyIDLNameSortPosition(aLhs.mProperty) <
+             nsCSSProps::PropertyIDLNameSortPosition(aRhs.mProperty);
+    }
+  };
+};
+
+/**
+ * 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
+{
+  nsCSSPropertyID mProperty;
+  StyleAnimationValue mValue;
+  float mOffset;
+  Maybe<ComputedTimingFunction> mTimingFunction;
+
+  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 IDL name.
+      int32_t order = nsCSSProps::PropertyIDLNameSortPosition(aLhs.mProperty) -
+                      nsCSSProps::PropertyIDLNameSortPosition(aRhs.mProperty);
+      if (order != 0) {
+        return order < 0;
+      }
+
+      // 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;
+  }
+};
+
+// ------------------------------------------------------------------
+//
+// Inlined helper methods
+//
+// ------------------------------------------------------------------
+
+inline bool
+IsInvalidValuePair(const PropertyValuePair& aPair, StyleBackendType aBackend)
+{
+  if (aBackend == StyleBackendType::Servo) {
+    return !aPair.mServoDeclarationBlock;
+  }
+
+  // There are three types of values we store as token streams:
+  //
+  // * Shorthand values (where we manually extract the token stream's string
+  //   value) and pass that along to various parsing methods
+  // * Longhand values with variable references
+  // * Invalid values
+  //
+  // We can distinguish between the last two cases because for invalid values
+  // we leave the token stream's mPropertyID as eCSSProperty_UNKNOWN.
+  return !nsCSSProps::IsShorthand(aPair.mProperty) &&
+         aPair.mValue.GetUnit() == eCSSUnit_TokenStream &&
+         aPair.mValue.GetTokenStreamValue()->mPropertyID
+           == eCSSProperty_UNKNOWN;
+}
+
+
+// ------------------------------------------------------------------
+//
+// Internal helper method declarations
+//
+// ------------------------------------------------------------------
+
+static void
+GetKeyframeListFromKeyframeSequence(JSContext* aCx,
+                                    nsIDocument* aDocument,
+                                    JS::ForOfIterator& aIterator,
+                                    nsTArray<Keyframe>& aResult,
+                                    ErrorResult& aRv);
+
+static bool
+ConvertKeyframeSequence(JSContext* aCx,
+                        nsIDocument* aDocument,
+                        JS::ForOfIterator& aIterator,
+                        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<nsString>& aValues);
+
+static bool
+AppendValueAsString(JSContext* aCx,
+                    nsTArray<nsString>& aValues,
+                    JS::Handle<JS::Value> aValue);
+
+static PropertyValuePair
+MakePropertyValuePair(nsCSSPropertyID aProperty, const nsAString& aStringValue,
+                      nsCSSParser& aParser, nsIDocument* aDocument);
+
+static bool
+HasValidOffsets(const nsTArray<Keyframe>& aKeyframes);
+
+static void
+MarkAsComputeValuesFailureKey(PropertyValuePair& aPair);
+
+static bool
+IsComputeValuesFailureKey(const PropertyValuePair& aPair);
+
+static void
+BuildSegmentsFromValueEntries(nsTArray<KeyframeValueEntry>& aEntries,
+                              nsTArray<AnimationProperty>& aResult);
+
+static void
+GetKeyframeListFromPropertyIndexedKeyframe(JSContext* aCx,
+                                           nsIDocument* aDocument,
+                                           JS::Handle<JS::Value> aValue,
+                                           nsTArray<Keyframe>& aResult,
+                                           ErrorResult& aRv);
+
+static bool
+RequiresAdditiveAnimation(const nsTArray<Keyframe>& aKeyframes,
+                          nsIDocument* aDocument);
+
+static void
+DistributeRange(const Range<Keyframe>& aSpacingRange,
+                const Range<Keyframe>& aRangeToAdjust);
+
+static void
+DistributeRange(const Range<Keyframe>& aSpacingRange);
+
+static void
+PaceRange(const Range<Keyframe>& aKeyframes,
+          const Range<double>& aCumulativeDistances);
+
+static nsTArray<double>
+GetCumulativeDistances(const nsTArray<ComputedKeyframeValues>& aValues,
+                       nsCSSPropertyID aProperty,
+                       nsStyleContext* aStyleContext);
+
+// ------------------------------------------------------------------
+//
+// Public API
+//
+// ------------------------------------------------------------------
+
+/* static */ nsTArray<Keyframe>
+KeyframeUtils::GetKeyframesFromObject(JSContext* aCx,
+                                      nsIDocument* aDocument,
+                                      JS::Handle<JSObject*> aFrames,
+                                      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, 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;
+  }
+
+  // We currently don't support additive animation. However, Web Animations
+  // says that if you don't have a keyframe at offset 0 or 1, then you should
+  // synthesize one using an additive zero value when you go to compose style.
+  // Until we implement additive animations we just throw if we encounter any
+  // set of keyframes that would put us in that situation.
+
+  if (RequiresAdditiveAnimation(keyframes, aDocument)) {
+    aRv.Throw(NS_ERROR_DOM_ANIM_MISSING_PROPS_ERR);
+    keyframes.Clear();
+  }
+
+  return keyframes;
+}
+
+/* static */ void
+KeyframeUtils::ApplySpacing(nsTArray<Keyframe>& aKeyframes,
+                            SpacingMode aSpacingMode,
+                            nsCSSPropertyID aProperty,
+                            nsTArray<ComputedKeyframeValues>& aComputedValues,
+                            nsStyleContext* aStyleContext)
+{
+  if (aKeyframes.IsEmpty()) {
+    return;
+  }
+
+  nsTArray<double> cumulativeDistances;
+  if (aSpacingMode == SpacingMode::paced) {
+    MOZ_ASSERT(IsAnimatableProperty(aProperty),
+               "Paced property should be animatable");
+
+    cumulativeDistances = GetCumulativeDistances(aComputedValues, aProperty,
+                                                 aStyleContext);
+    // Reset the computed offsets if using paced spacing.
+    for (Keyframe& keyframe : aKeyframes) {
+      keyframe.mComputedOffset = Keyframe::kComputedOffsetNotSet;
+    }
+  }
+
+  // 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.cend() - 1;
+  const RangedPtr<Keyframe> begin(aKeyframes.begin(), 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).
+    if (aSpacingMode == SpacingMode::distribute) {
+      DistributeRange(Range<Keyframe>(keyframeA, keyframeB + 1));
+    } else {
+      // a) Find Paced A (first paceable keyframe) and
+      //    Paced B (last paceable keyframe) in [keyframe A, keyframe B].
+      RangedPtr<Keyframe> pacedA = keyframeA;
+      while (pacedA < keyframeB &&
+             cumulativeDistances[pacedA - begin] == kNotPaceable) {
+        ++pacedA;
+      }
+      RangedPtr<Keyframe> pacedB = keyframeB;
+      while (pacedB > keyframeA &&
+             cumulativeDistances[pacedB - begin] == kNotPaceable) {
+        --pacedB;
+      }
+      // As spec says, if there is no paceable keyframe
+      // in [keyframe A, keyframe B], we let Paced A and Paced B refer to
+      // keyframe B.
+      if (pacedA > pacedB) {
+        pacedA = pacedB = keyframeB;
+      }
+      // b) Apply distributing offsets in (keyframe A, Paced A] and
+      //    [Paced B, keyframe B).
+      DistributeRange(Range<Keyframe>(keyframeA, keyframeB + 1),
+                      Range<Keyframe>(keyframeA + 1, pacedA + 1));
+      DistributeRange(Range<Keyframe>(keyframeA, keyframeB + 1),
+                      Range<Keyframe>(pacedB, keyframeB));
+      // c) Apply paced offsets to each paceable keyframe in (Paced A, Paced B).
+      //    We pass the range [Paced A, Paced B] since PaceRange needs the end
+      //    points of the range in order to calculate the correct offset.
+      PaceRange(Range<Keyframe>(pacedA, pacedB + 1),
+                Range<double>(&cumulativeDistances[pacedA - begin],
+                              pacedB - pacedA + 1));
+      // d) Fill in any computed offsets in (Paced A, Paced B) that are still
+      //    not set (e.g. because the keyframe was not paceable, or because the
+      //    cumulative distance between paceable properties was zero).
+      for (RangedPtr<Keyframe> frame = pacedA + 1; frame < pacedB; ++frame) {
+        if (frame->mComputedOffset != Keyframe::kComputedOffsetNotSet) {
+          continue;
+        }
+
+        RangedPtr<Keyframe> start = frame - 1;
+        RangedPtr<Keyframe> end = frame + 1;
+        while (end < pacedB &&
+               end->mComputedOffset == Keyframe::kComputedOffsetNotSet) {
+          ++end;
+        }
+        DistributeRange(Range<Keyframe>(start, end + 1));
+        frame = end;
+      }
+    }
+    keyframeA = keyframeB;
+  }
+}
+
+/* static */ void
+KeyframeUtils::ApplyDistributeSpacing(nsTArray<Keyframe>& aKeyframes)
+{
+  nsTArray<ComputedKeyframeValues> emptyArray;
+  ApplySpacing(aKeyframes, SpacingMode::distribute, eCSSProperty_UNKNOWN,
+               emptyArray, nullptr);
+}
+
+/* static */ nsTArray<ComputedKeyframeValues>
+KeyframeUtils::GetComputedKeyframeValues(const nsTArray<Keyframe>& aKeyframes,
+                                         dom::Element* aElement,
+                                         nsStyleContext* aStyleContext)
+{
+  MOZ_ASSERT(aStyleContext);
+  MOZ_ASSERT(aElement);
+
+  StyleBackendType styleBackend = aElement->OwnerDoc()->GetStyleBackendType();
+
+  const size_t len = aKeyframes.Length();
+  nsTArray<ComputedKeyframeValues> result(len);
+
+  for (const Keyframe& frame : aKeyframes) {
+    nsCSSPropertyIDSet propertiesOnThisKeyframe;
+    ComputedKeyframeValues* computedValues = result.AppendElement();
+    for (const PropertyValuePair& pair :
+           PropertyPriorityIterator(frame.mPropertyValues)) {
+      MOZ_ASSERT(!pair.mServoDeclarationBlock ||
+                 styleBackend == StyleBackendType::Servo,
+                 "Animation values were parsed using Servo backend but target"
+                 " element is not using Servo backend?");
+
+      if (IsInvalidValuePair(pair, styleBackend)) {
+        continue;
+      }
+
+      // Expand each value into the set of longhands and produce
+      // a KeyframeValueEntry for each value.
+      nsTArray<PropertyStyleAnimationValuePair> values;
+
+      if (styleBackend == StyleBackendType::Servo) {
+        if (!StyleAnimationValue::ComputeValues(pair.mProperty,
+              CSSEnabledState::eForAllContent, aStyleContext,
+              *pair.mServoDeclarationBlock, values)) {
+          continue;
+        }
+      } else {
+        // For shorthands, we store the string as a token stream so we need to
+        // extract that first.
+        if (nsCSSProps::IsShorthand(pair.mProperty)) {
+          nsCSSValueTokenStream* tokenStream = pair.mValue.GetTokenStreamValue();
+          if (!StyleAnimationValue::ComputeValues(pair.mProperty,
+                CSSEnabledState::eForAllContent, aElement, aStyleContext,
+                tokenStream->mTokenStream, /* aUseSVGMode */ false, values) ||
+              IsComputeValuesFailureKey(pair)) {
+            continue;
+          }
+        } else {
+          if (!StyleAnimationValue::ComputeValues(pair.mProperty,
+                CSSEnabledState::eForAllContent, aElement, aStyleContext,
+                pair.mValue, /* aUseSVGMode */ false, values)) {
+            continue;
+          }
+          MOZ_ASSERT(values.Length() == 1,
+                    "Longhand properties should produce a single"
+                    " StyleAnimationValue");
+        }
+      }
+
+      for (auto& value : values) {
+        // If we already got a value for this property on the keyframe,
+        // skip this one.
+        if (propertiesOnThisKeyframe.HasProperty(value.mProperty)) {
+          continue;
+        }
+        computedValues->AppendElement(value);
+        propertiesOnThisKeyframe.AddProperty(value.mProperty);
+      }
+    }
+  }
+
+  MOZ_ASSERT(result.Length() == aKeyframes.Length(), "Array length mismatch");
+  return result;
+}
+
+/* static */ nsTArray<AnimationProperty>
+KeyframeUtils::GetAnimationPropertiesFromKeyframes(
+  const nsTArray<Keyframe>& aKeyframes,
+  const nsTArray<ComputedKeyframeValues>& aComputedValues,
+  nsStyleContext* aStyleContext)
+{
+  MOZ_ASSERT(aKeyframes.Length() == aComputedValues.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 : aComputedValues[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;
+    }
+  }
+
+  nsTArray<AnimationProperty> result;
+  BuildSegmentsFromValueEntries(entries, result);
+  return result;
+}
+
+/* static */ bool
+KeyframeUtils::IsAnimatableProperty(nsCSSPropertyID aProperty)
+{
+  if (aProperty == eCSSProperty_UNKNOWN) {
+    return false;
+  }
+
+  if (!nsCSSProps::IsShorthand(aProperty)) {
+    return nsCSSProps::kAnimTypeTable[aProperty] != eStyleAnimType_None;
+  }
+
+  CSSPROPS_FOR_SHORTHAND_SUBPROPERTIES(subprop, aProperty,
+                                       CSSEnabledState::eForAllContent) {
+    if (nsCSSProps::kAnimTypeTable[*subprop] != eStyleAnimType_None) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+// ------------------------------------------------------------------
+//
+// 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 aRv Out param to store any errors thrown by this function.
+ */
+static void
+GetKeyframeListFromKeyframeSequence(JSContext* aCx,
+                                    nsIDocument* aDocument,
+                                    JS::ForOfIterator& aIterator,
+                                    nsTArray<Keyframe>& aResult,
+                                    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, aResult)) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    aResult.Clear();
+    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)) {
+    aRv.ThrowTypeError<dom::MSG_INVALID_KEYFRAME_OFFSETS>();
+    aResult.Clear();
+    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,
+                        nsIDocument* aDocument,
+                        JS::ForOfIterator& aIterator,
+                        nsTArray<Keyframe>& aResult)
+{
+  JS::Rooted<JS::Value> value(aCx);
+  nsCSSParser parser(aDocument->CSSLoader());
+
+  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(aCx, dom::MSG_NOT_OBJECT,
+                             "Element of sequence<Keyframe> argument");
+      return false;
+    }
+
+    // Convert the JS value into a BaseKeyframe dictionary value.
+    dom::binding_detail::FastBaseKeyframe keyframeDict;
+    if (!keyframeDict.Init(aCx, value,
+                           "Element of sequence<Keyframe> argument")) {
+      return false;
+    }
+
+    Keyframe* keyframe = aResult.AppendElement(fallible);
+    if (!keyframe) {
+      return false;
+    }
+    if (!keyframeDict.mOffset.IsNull()) {
+      keyframe->mOffset.emplace(keyframeDict.mOffset.Value());
+    }
+
+    ErrorResult rv;
+    keyframe->mTimingFunction =
+      TimingParams::ParseEasing(keyframeDict.mEasing, aDocument, rv);
+    if (rv.MaybeSetPendingException(aCx)) {
+      return false;
+    }
+
+    // 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;
+      }
+    }
+
+    for (PropertyValuesPair& pair : propertyValuePairs) {
+      MOZ_ASSERT(pair.mValues.Length() == 1);
+      keyframe->mPropertyValues.AppendElement(
+        MakePropertyValuePair(pair.mProperty, pair.mValues[0], parser,
+                              aDocument));
+
+      // 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) &&
+          keyframeDict.mSimulateComputeValuesFailure) {
+        MarkAsComputeValuesFailureKey(keyframe->mPropertyValues.LastElement());
+      }
+    }
+  }
+
+  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++) {
+    nsAutoJSString propName;
+    if (!propName.init(aCx, ids[i])) {
+      return false;
+    }
+    nsCSSPropertyID property =
+      nsCSSProps::LookupPropertyByIDLName(propName,
+                                          CSSEnabledState::eForAllContent);
+    if (KeyframeUtils::IsAnimatableProperty(property)) {
+      AdditionalProperty* p = properties.AppendElement();
+      p->mProperty = property;
+      p->mJsidIndex = 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<nsString>& 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<nsString>& aValues,
+                    JS::Handle<JS::Value> aValue)
+{
+  return ConvertJSValueToString(aCx, aValue, dom::eStringify, dom::eStringify,
+                                *aValues.AppendElement());
+}
+
+/**
+ * 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 aParser The CSS parser object to use.
+ * @param aDocument The document to use when parsing.
+ * @return The constructed PropertyValuePair object.
+ */
+static PropertyValuePair
+MakePropertyValuePair(nsCSSPropertyID aProperty, const nsAString& aStringValue,
+                      nsCSSParser& aParser, nsIDocument* aDocument)
+{
+  MOZ_ASSERT(aDocument);
+  PropertyValuePair result;
+
+  result.mProperty = aProperty;
+
+  if (aDocument->GetStyleBackendType() == StyleBackendType::Servo) {
+    nsCString name = nsCSSProps::GetStringValue(aProperty);
+
+    NS_ConvertUTF16toUTF8 value(aStringValue);
+    RefPtr<ThreadSafeURIHolder> base =
+      new ThreadSafeURIHolder(aDocument->GetDocumentURI());
+    RefPtr<ThreadSafeURIHolder> referrer =
+      new ThreadSafeURIHolder(aDocument->GetDocumentURI());
+    RefPtr<ThreadSafePrincipalHolder> principal =
+      new ThreadSafePrincipalHolder(aDocument->NodePrincipal());
+
+    nsCString baseString;
+    aDocument->GetDocumentURI()->GetSpec(baseString);
+
+    RefPtr<RawServoDeclarationBlock> servoDeclarationBlock =
+      Servo_ParseProperty(&name, &value, &baseString,
+                          base, referrer, principal).Consume();
+
+    if (servoDeclarationBlock) {
+      result.mServoDeclarationBlock = servoDeclarationBlock.forget();
+      return result;
+    }
+  }
+
+  nsCSSValue value;
+  if (!nsCSSProps::IsShorthand(aProperty)) {
+    aParser.ParseLonghandProperty(aProperty,
+                                  aStringValue,
+                                  aDocument->GetDocumentURI(),
+                                  aDocument->GetDocumentURI(),
+                                  aDocument->NodePrincipal(),
+                                  value);
+  }
+
+  if (value.GetUnit() == eCSSUnit_Null) {
+    // Either we have a shorthand, or we failed to parse a longhand.
+    // In either case, store the string value as a token stream.
+    nsCSSValueTokenStream* tokenStream = new nsCSSValueTokenStream;
+    tokenStream->mTokenStream = aStringValue;
+
+    // We are about to convert a null value to a token stream value but
+    // by leaving the mPropertyID as unknown, we will be able to
+    // distinguish between invalid values and valid token stream values
+    // (e.g. values with variable references).
+    MOZ_ASSERT(tokenStream->mPropertyID == eCSSProperty_UNKNOWN,
+               "The property of a token stream should be initialized"
+               " to unknown");
+
+    // By leaving mShorthandPropertyID as unknown, we ensure that when
+    // we call nsCSSValue::AppendToString we get back the string stored
+    // in mTokenStream.
+    MOZ_ASSERT(tokenStream->mShorthandPropertyID == eCSSProperty_UNKNOWN,
+               "The shorthand property of a token stream should be initialized"
+               " to unknown");
+    value.SetTokenStreamValue(tokenStream);
+  } else {
+    // If we succeeded in parsing with Gecko, but not Servo the animation is
+    // not going to work since, for the purposes of animation, we're going to
+    // ignore |mValue| when the backend is Servo.
+    NS_WARNING_ASSERTION(aDocument->GetStyleBackendType() !=
+                           StyleBackendType::Servo,
+                         "Gecko succeeded in parsing where Servo failed");
+  }
+
+  result.mValue = value;
+
+  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;
+}
+
+/**
+ * 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),
+             "Only shorthand property values can be marked as failure values");
+
+  // We store shorthand values as nsCSSValueTokenStream objects whose mProperty
+  // and mShorthandPropertyID are eCSSProperty_UNKNOWN and whose mTokenStream
+  // member contains the shorthand property's value as a string.
+  //
+  // We need to leave mShorthandPropertyID as eCSSProperty_UNKNOWN so that
+  // nsCSSValue::AppendToString returns the mTokenStream value, but we can
+  // update mPropertyID to a special value to indicate that this is
+  // a special failure sentinel.
+  nsCSSValueTokenStream* tokenStream = aPair.mValue.GetTokenStreamValue();
+  MOZ_ASSERT(tokenStream->mPropertyID == eCSSProperty_UNKNOWN,
+             "Shorthand value should initially have an unknown property ID");
+  tokenStream->mPropertyID = eCSSPropertyExtra_no_properties;
+}
+
+/**
+ * Returns true if |aPair| is a property-value pair on which we have
+ * previously called MarkAsComputeValuesFailureKey (and hence we should
+ * simulate failure when calling StyleAnimationValue::ComputeValues using its
+ * value).
+ *
+ * @param aPair The property-value pair to test.
+ * @return True if |aPair| represents a failure value.
+ */
+static bool
+IsComputeValuesFailureKey(const PropertyValuePair& aPair)
+{
+  return nsCSSProps::IsShorthand(aPair.mProperty) &&
+         aPair.mValue.GetTokenStreamValue()->mPropertyID ==
+           eCSSPropertyExtra_no_properties;
+}
+
+/**
+ * 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, since we don't yet implement additive animation and hence
+  // don't have sensible fallback behavior when these values are missing, the
+  // following loop takes care to identify properties that lack a value at
+  // offset 0.0/1.0 and drops those properties from |aResult|.
+
+  nsCSSPropertyID lastProperty = eCSSProperty_UNKNOWN;
+  AnimationProperty* animationProperty = nullptr;
+
+  size_t i = 0, n = aEntries.Length();
+
+  while (i < n) {
+    // Check that the last property ends with an entry at offset 1.
+    if (i + 1 == n) {
+      if (aEntries[i].mOffset != 1.0f && animationProperty) {
+        aResult.RemoveElementAt(aResult.Length() - 1);
+        animationProperty = nullptr;
+      }
+      break;
+    }
+
+    MOZ_ASSERT(aEntries[i].mProperty != eCSSProperty_UNKNOWN &&
+               aEntries[i + 1].mProperty != eCSSProperty_UNKNOWN,
+               "Each entry should specify a valid property");
+
+    // Skip properties that don't have an entry with offset 0.
+    if (aEntries[i].mProperty != lastProperty &&
+        aEntries[i].mOffset != 0.0f) {
+      // 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.
+      ++i;
+      continue;
+    }
+
+    // Drop properties that don't end with an entry with offset 1.
+    if (aEntries[i].mProperty != aEntries[i + 1].mProperty &&
+        aEntries[i].mOffset != 1.0f) {
+      if (animationProperty) {
+        aResult.RemoveElementAt(aResult.Length() - 1);
+        animationProperty = nullptr;
+      }
+      ++i;
+      continue;
+    }
+
+    // Starting from i, determine the next [i, j] interval from which to
+    // generate a segment.
+    size_t j;
+    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);
+      j = i + 1;
+      while (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.
+        j = i + 1;
+        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;
+      }
+    } else {
+      while (aEntries[i].mOffset == aEntries[i + 1].mOffset &&
+             aEntries[i].mProperty == aEntries[i + 1].mProperty) {
+        ++i;
+      }
+      j = i + 1;
+    }
+
+    // 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;
+
+    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,
+                                           nsIDocument* 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;
+  if (!keyframeDict.Init(aCx, aValue, "BasePropertyIndexedKeyframe argument",
+                         false)) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return;
+  }
+
+  Maybe<ComputedTimingFunction> easing =
+    TimingParams::ParseEasing(keyframeDict.mEasing, aDocument, aRv);
+  if (aRv.Failed()) {
+    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.
+  nsCSSParser parser(aDocument->CSSLoader());
+  nsClassHashtable<nsFloatHashKey, Keyframe> processedKeyframes;
+  for (const PropertyValuesPair& pair : propertyValuesPairs) {
+    size_t count = pair.mValues.Length();
+    if (count == 0) {
+      // No animation values for this property.
+      continue;
+    }
+    if (count == 1) {
+      // We don't support additive values and so can't support an
+      // animation that goes from the underlying value to this
+      // specified value.  Throw an exception until we do support this.
+      aRv.Throw(NS_ERROR_DOM_ANIM_MISSING_PROPS_ERR);
+      return;
+    }
+
+    size_t n = pair.mValues.Length() - 1;
+    size_t i = 0;
+
+    for (const nsString& stringValue : pair.mValues) {
+      double offset = i++ / double(n);
+      Keyframe* keyframe = processedKeyframes.LookupOrAdd(offset);
+      if (keyframe->mPropertyValues.IsEmpty()) {
+        keyframe->mTimingFunction = easing;
+        keyframe->mComputedOffset = offset;
+      }
+      keyframe->mPropertyValues.AppendElement(
+        MakePropertyValuePair(pair.mProperty, stringValue, parser, aDocument));
+    }
+  }
+
+  aResult.SetCapacity(processedKeyframes.Count());
+  for (auto iter = processedKeyframes.Iter(); !iter.Done(); iter.Next()) {
+    aResult.AppendElement(Move(*iter.UserData()));
+  }
+
+  aResult.Sort(ComputedOffsetComparator());
+}
+
+/**
+ * Returns true if the supplied set of keyframes has keyframe values for
+ * any property for which it does not also supply a value for the 0% and 100%
+ * offsets. In this case we are supposed to synthesize an additive zero value
+ * but since we don't support additive animation yet we can't support this
+ * case. We try to detect that here so we can throw an exception. The check is
+ * not entirely accurate but should detect most common cases.
+ *
+ * @param aKeyframes The set of keyframes to analyze.
+ * @param aDocument The document to use when parsing keyframes so we can
+ *   try to detect where we have an invalid value at 0%/100%.
+ */
+static bool
+RequiresAdditiveAnimation(const nsTArray<Keyframe>& aKeyframes,
+                          nsIDocument* aDocument)
+{
+  // We are looking to see if that every property referenced in |aKeyframes|
+  // has a valid property at offset 0.0 and 1.0. The check as to whether a
+  // property is valid or not, however, is not precise. We only check if the
+  // property can be parsed, NOT whether it can also be converted to a
+  // StyleAnimationValue since doing that requires a target element bound to
+  // a document which we might not always have at the point where we want to
+  // perform this check.
+  //
+  // This is only a temporary measure until we implement additive animation.
+  // So as long as this check catches most cases, and we don't do anything
+  // horrible in one of the cases we can't detect, it should be sufficient.
+
+  nsCSSPropertyIDSet properties;              // All properties encountered.
+  nsCSSPropertyIDSet propertiesWithFromValue; // Those with a defined 0% value.
+  nsCSSPropertyIDSet propertiesWithToValue;   // Those with a defined 100% value.
+
+  auto addToPropertySets = [&](nsCSSPropertyID aProperty, double aOffset) {
+    properties.AddProperty(aProperty);
+    if (aOffset == 0.0) {
+      propertiesWithFromValue.AddProperty(aProperty);
+    } else if (aOffset == 1.0) {
+      propertiesWithToValue.AddProperty(aProperty);
+    }
+  };
+
+  StyleBackendType styleBackend = aDocument->GetStyleBackendType();
+
+  for (size_t i = 0, len = aKeyframes.Length(); i < len; i++) {
+    const Keyframe& frame = aKeyframes[i];
+
+    // We won't have called ApplySpacing when this is called so
+    // we can't use frame.mComputedOffset. Instead we do a rough version
+    // of that algorithm that substitutes null offsets with 0.0 for the first
+    // frame, 1.0 for the last frame, and 0.5 for everything else.
+    double computedOffset = i == len - 1
+                            ? 1.0
+                            : i == 0 ? 0.0 : 0.5;
+    double offsetToUse = frame.mOffset
+                         ? frame.mOffset.value()
+                         : computedOffset;
+
+    for (const PropertyValuePair& pair : frame.mPropertyValues) {
+      if (IsInvalidValuePair(pair, styleBackend)) {
+        continue;
+      }
+
+      if (nsCSSProps::IsShorthand(pair.mProperty)) {
+        if (styleBackend == StyleBackendType::Gecko) {
+          nsCSSValueTokenStream* tokenStream =
+            pair.mValue.GetTokenStreamValue();
+          nsCSSParser parser(aDocument->CSSLoader());
+          if (!parser.IsValueValidForProperty(pair.mProperty,
+                                              tokenStream->mTokenStream)) {
+            continue;
+          }
+        }
+        // For the Servo backend, invalid shorthand values are represented by
+        // a null mServoDeclarationBlock member which we skip above in
+        // IsInvalidValuePair.
+        MOZ_ASSERT(styleBackend != StyleBackendType::Servo ||
+                   pair.mServoDeclarationBlock);
+        CSSPROPS_FOR_SHORTHAND_SUBPROPERTIES(
+            prop, pair.mProperty, CSSEnabledState::eForAllContent) {
+          addToPropertySets(*prop, offsetToUse);
+        }
+      } else {
+        addToPropertySets(pair.mProperty, offsetToUse);
+      }
+    }
+  }
+
+  return !propertiesWithFromValue.Equals(properties) ||
+         !propertiesWithToValue.Equals(properties);
+}
+
+/**
+ * Evenly distribute the computed offsets in (A, B).
+ * We pass the range keyframes in [A, B] and use A, B to calculate distributing
+ * computed offsets in (A, B). The second range, aRangeToAdjust, is passed, so
+ * we can know which keyframe we want to apply to. aRangeToAdjust should be in
+ * the range of aSpacingRange.
+ *
+ * @param aSpacingRange The sequence of keyframes between whose endpoints we
+ *   should apply distribute spacing.
+ * @param aRangeToAdjust The range of keyframes we want to apply to.
+ */
+static void
+DistributeRange(const Range<Keyframe>& aSpacingRange,
+                const Range<Keyframe>& aRangeToAdjust)
+{
+  MOZ_ASSERT(aRangeToAdjust.begin() >= aSpacingRange.begin() &&
+             aRangeToAdjust.end() <= aSpacingRange.end(),
+             "Out of range");
+  const size_t n = aSpacingRange.length() - 1;
+  const double startOffset = aSpacingRange[0].mComputedOffset;
+  const double diffOffset = aSpacingRange[n].mComputedOffset - startOffset;
+  for (auto iter = aRangeToAdjust.begin();
+       iter != aRangeToAdjust.end();
+       ++iter) {
+    size_t index = iter - aSpacingRange.begin();
+    iter->mComputedOffset = startOffset + double(index) / n * diffOffset;
+  }
+}
+
+/**
+ * Overload of DistributeRange to apply distribute spacing to all keyframes in
+ * between the endpoints of the given range.
+ *
+ * @param aSpacingRange The sequence of keyframes between whose endpoints we
+ *   should apply distribute spacing.
+ */
+static void
+DistributeRange(const Range<Keyframe>& aSpacingRange)
+{
+  // We don't need to apply distribute spacing to keyframe A and keyframe B.
+  DistributeRange(aSpacingRange,
+                  Range<Keyframe>(aSpacingRange.begin() + 1,
+                                  aSpacingRange.end() - 1));
+}
+
+/**
+ * Apply paced spacing to all paceable keyframes in between the endpoints of the
+ * given range.
+ *
+ * @param aKeyframes The range of keyframes between whose endpoints we should
+ *   apply paced spacing. Both endpoints should be paceable, i.e. the
+ *   corresponding elements in |aCumulativeDist| should not be kNotPaceable.
+ *   Within this function, we refer to the start and end points of this range
+ *   as Paced A and Paced B respectively in keeping with the notation used in
+ *   the spec.
+ * @param aCumulativeDistances The sequence of cumulative distances of the paced
+ *   property as returned by GetCumulativeDistances(). This acts as a
+ *   parallel range to |aKeyframes|.
+ */
+static void
+PaceRange(const Range<Keyframe>& aKeyframes,
+          const Range<double>& aCumulativeDistances)
+{
+  MOZ_ASSERT(aKeyframes.length() == aCumulativeDistances.length(),
+             "Range length mismatch");
+
+  const size_t len = aKeyframes.length();
+  // If there is nothing between the end points, there is nothing to space.
+  if (len < 3) {
+    return;
+  }
+
+  const double distA = *(aCumulativeDistances.begin());
+  const double distB = *(aCumulativeDistances.end() - 1);
+  MOZ_ASSERT(distA != kNotPaceable && distB != kNotPaceable,
+             "Both Paced A and Paced B should be paceable");
+
+  // If the total distance is zero, we should fall back to distribute spacing.
+  // The caller will fill-in any keyframes without a computed offset using
+  // distribute spacing so we can just return here.
+  if (distA == distB) {
+    return;
+  }
+
+  const RangedPtr<Keyframe> pacedA = aKeyframes.begin();
+  const RangedPtr<Keyframe> pacedB = aKeyframes.end() - 1;
+  MOZ_ASSERT(pacedA->mComputedOffset != Keyframe::kComputedOffsetNotSet &&
+             pacedB->mComputedOffset != Keyframe::kComputedOffsetNotSet,
+             "Both Paced A and Paced B should have valid computed offsets");
+
+  // Apply computed offset.
+  const double offsetA     = pacedA->mComputedOffset;
+  const double diffOffset  = pacedB->mComputedOffset - offsetA;
+  const double initialDist = distA;
+  const double totalDist   = distB - initialDist;
+  for (auto iter = pacedA + 1; iter != pacedB; ++iter) {
+    size_t k = iter - aKeyframes.begin();
+    if (aCumulativeDistances[k] == kNotPaceable) {
+      continue;
+    }
+
+    double dist = aCumulativeDistances[k] - initialDist;
+    iter->mComputedOffset = offsetA + diffOffset * dist / totalDist;
+  }
+}
+
+/**
+ * Get cumulative distances for the paced property.
+ *
+ * @param aValues The computed values returned by GetComputedKeyframeValues.
+ * @param aPacedProperty The paced property.
+ * @param aStyleContext The style context for computing distance on transform.
+ * @return The cumulative distances for the paced property. The length will be
+ *   the same as aValues.
+ */
+static nsTArray<double>
+GetCumulativeDistances(const nsTArray<ComputedKeyframeValues>& aValues,
+                       nsCSSPropertyID aPacedProperty,
+                       nsStyleContext* aStyleContext)
+{
+  // a) If aPacedProperty is a shorthand property, get its components.
+  //    Otherwise, just add the longhand property into the set.
+  size_t pacedPropertyCount = 0;
+  nsCSSPropertyIDSet pacedPropertySet;
+  bool isShorthand = nsCSSProps::IsShorthand(aPacedProperty);
+  if (isShorthand) {
+    CSSPROPS_FOR_SHORTHAND_SUBPROPERTIES(p, aPacedProperty,
+                                         CSSEnabledState::eForAllContent) {
+      pacedPropertySet.AddProperty(*p);
+      ++pacedPropertyCount;
+    }
+  } else {
+    pacedPropertySet.AddProperty(aPacedProperty);
+    pacedPropertyCount = 1;
+  }
+
+  // b) Search each component (shorthand) or the longhand property, and
+  //    calculate the cumulative distances of paceable keyframe pairs.
+  const size_t len = aValues.Length();
+  nsTArray<double> cumulativeDistances(len);
+  // cumulativeDistances is a parallel array to |aValues|, so set its length to
+  // the length of |aValues|.
+  cumulativeDistances.SetLength(len);
+  ComputedKeyframeValues prevPacedValues;
+  size_t preIdx = 0;
+  for (size_t i = 0; i < len; ++i) {
+    // Find computed values of the paced property.
+    ComputedKeyframeValues pacedValues;
+    for (const PropertyStyleAnimationValuePair& pair : aValues[i]) {
+      if (pacedPropertySet.HasProperty(pair.mProperty)) {
+        pacedValues.AppendElement(pair);
+      }
+    }
+
+    // Check we have values for all the paceable longhand components.
+    if (pacedValues.Length() != pacedPropertyCount) {
+      // This keyframe is not paceable, assign kNotPaceable and skip it.
+      cumulativeDistances[i] = kNotPaceable;
+      continue;
+    }
+
+    // Sort the pacedValues first, so the order of subproperties of
+    // pacedValues is always the same as that of prevPacedValues.
+    if (isShorthand) {
+      pacedValues.Sort(
+        TPropertyPriorityComparator<PropertyStyleAnimationValuePair>());
+    }
+
+    if (prevPacedValues.IsEmpty()) {
+      // This is the first paceable keyframe so its cumulative distance is 0.0.
+      cumulativeDistances[i] = 0.0;
+    } else {
+      double dist = 0.0;
+      if (isShorthand) {
+        // Apply the distance by the square root of the sum of squares of
+        // longhand component distances.
+        for (size_t propIdx = 0; propIdx < pacedPropertyCount; ++propIdx) {
+          nsCSSPropertyID prop = prevPacedValues[propIdx].mProperty;
+          MOZ_ASSERT(pacedValues[propIdx].mProperty == prop,
+                     "Property mismatch");
+
+          double componentDistance = 0.0;
+          if (StyleAnimationValue::ComputeDistance(
+                prop,
+                prevPacedValues[propIdx].mValue,
+                pacedValues[propIdx].mValue,
+                aStyleContext,
+                componentDistance)) {
+            dist += componentDistance * componentDistance;
+          }
+        }
+        dist = sqrt(dist);
+      } else {
+        // If the property is longhand, we just use the 1st value.
+        // If ComputeDistance() fails, |dist| will remain zero so there will be
+        // no distance between the previous paced value and this value.
+        Unused <<
+          StyleAnimationValue::ComputeDistance(aPacedProperty,
+                                               prevPacedValues[0].mValue,
+                                               pacedValues[0].mValue,
+                                               aStyleContext,
+                                               dist);
+      }
+      cumulativeDistances[i] = cumulativeDistances[preIdx] + dist;
+    }
+    prevPacedValues.SwapElements(pacedValues);
+    preIdx = i;
+  }
+  return cumulativeDistances;
+}
+
+} // namespace mozilla