--- a/dom/animation/KeyframeEffectReadOnly.cpp
+++ b/dom/animation/KeyframeEffectReadOnly.cpp
@@ -650,33 +650,30 @@ KeyframeEffectReadOnly::ComposeStyleRule
     aStyleRule->AddValue(aProperty.mProperty, Move(val));
   } else if (valuePosition < 0.5) {
     aStyleRule->AddValue(aProperty.mProperty, Move(fromValue));
   } else {
     aStyleRule->AddValue(aProperty.mProperty, Move(toValue));
   }
 }
 
-// Bug 1333311 - We use two branches for Gecko and Stylo. However, it's
-// better to remove the duplicated code.
 void
 KeyframeEffectReadOnly::ComposeStyleRule(
   RawServoAnimationValueMap& aAnimationValues,
   const AnimationProperty& aProperty,
   const AnimationPropertySegment& aSegment,
   const ComputedTiming& aComputedTiming)
 {
-  // Bug 1329878 - Stylo: Implement accumulate and addition on Servo
-  // AnimationValue.
-
   Servo_AnimationCompose(&aAnimationValues,
                          &mBaseStyleValuesForServo,
                          aProperty.mProperty,
                          &aSegment,
-                         &aComputedTiming);
+                         &aProperty.mSegments.LastElement(),
+                         &aComputedTiming,
+                         mEffectOptions.mIterationComposite);
 }
 
 template<typename ComposeAnimationResult>
 void
 KeyframeEffectReadOnly::ComposeStyle(
   ComposeAnimationResult&& aComposeResult,
   const nsCSSPropertyIDSet& aPropertiesToSkip)
 {

--- a/layout/reftests/web-animations/reftest.list
+++ b/layout/reftests/web-animations/reftest.list
@@ -13,10 +13,10 @@ test-pref(dom.animations-api.core.enable
 test-pref(dom.animations-api.core.enabled,true) == stacking-context-transform-changing-keyframe-in-delay.html stacking-context-animation-ref.html
 test-pref(dom.animations-api.core.enabled,true) == stacking-context-transform-changing-target.html stacking-context-animation-changing-target-ref.html
 
 test-pref(dom.animations-api.core.enabled,true) == stacking-context-transform-changing-target-in-delay.html stacking-context-animation-changing-target-ref.html
 test-pref(dom.animations-api.core.enabled,true) == stacking-context-transform-changing-effect.html stacking-context-animation-ref.html
 test-pref(dom.animations-api.core.enabled,true) == stacking-context-transform-changing-display-property.html stacking-context-animation-ref.html
 test-pref(dom.animations-api.core.enabled,true) == stacking-context-transform-losing-css-animation-in-delay.html stacking-context-animation-ref.html
 test-pref(dom.animations-api.core.enabled,true) == style-updates-on-iteration-composition-changed-from-accumulate-to-replace.html style-updates-for-iteration-composite-ref.html
-test-pref(dom.animations-api.core.enabled,true) fails-if(stylo) == style-updates-on-iteration-composition-changed-from-replace-to-accumulate.html style-updates-for-iteration-composite-ref.html
-test-pref(dom.animations-api.core.enabled,true) fails-if(stylo) == style-updates-on-current-iteration-changed.html style-updates-for-iteration-composite-ref.html
+test-pref(dom.animations-api.core.enabled,true) == style-updates-on-iteration-composition-changed-from-replace-to-accumulate.html style-updates-for-iteration-composite-ref.html
+test-pref(dom.animations-api.core.enabled,true) == style-updates-on-current-iteration-changed.html style-updates-for-iteration-composite-ref.html

--- a/layout/style/ServoBindingList.h
+++ b/layout/style/ServoBindingList.h
@@ -311,17 +311,19 @@ SERVO_BINDING_FUNC(Servo_DeclarationBloc
 // Compose animation value for a given property.
 // |base_values| is nsRefPtrHashtable<nsUint32HashKey, RawServoAnimationValue>.
 // We use void* to avoid exposing nsRefPtrHashtable in FFI.
 SERVO_BINDING_FUNC(Servo_AnimationCompose, void,
                    RawServoAnimationValueMapBorrowedMut animation_values,
                    void* base_values,
                    nsCSSPropertyID property,
                    RawGeckoAnimationPropertySegmentBorrowed animation_segment,
-                   RawGeckoComputedTimingBorrowed computed_timing)
+                   RawGeckoAnimationPropertySegmentBorrowed last_segment,
+                   RawGeckoComputedTimingBorrowed computed_timing,
+                   mozilla::dom::IterationCompositeOperation iteration_composite)
 
 // presentation attributes
 SERVO_BINDING_FUNC(Servo_DeclarationBlock_PropertyIsSet, bool,
                    RawServoDeclarationBlockBorrowed declarations,
                    nsCSSPropertyID property)
 SERVO_BINDING_FUNC(Servo_DeclarationBlock_SetIdentStringValue, void,
                    RawServoDeclarationBlockBorrowed declarations,
                    nsCSSPropertyID property,

--- a/layout/style/ServoBindings.h
+++ b/layout/style/ServoBindings.h
@@ -38,16 +38,19 @@ namespace mozilla {
   enum class CSSPseudoElementType : uint8_t;
   struct Keyframe;
   enum Side;
   struct StyleTransition;
   namespace css {
     struct URLValue;
     struct ImageValue;
   };
+  namespace dom {
+    enum class IterationCompositeOperation : uint8_t;
+  };
   enum class UpdateAnimationsTasks : uint8_t;
   struct LangGroupFontPrefs;
   class ServoStyleSheet;
   class ServoElementSnapshotTable;
 }
 using mozilla::FontFamilyList;
 using mozilla::FontFamilyType;
 using mozilla::ServoElementSnapshot;

--- a/layout/style/ServoBindings.toml
+++ b/layout/style/ServoBindings.toml
@@ -43,16 +43,17 @@ args = [
 [structs]
 headers = [
     "nsStyleStruct.h",
     "mozilla/ServoPropPrefList.h",
     "mozilla/StyleAnimationValue.h",
     "gfxFontConstants.h",
     "nsThemeConstants.h",
     "mozilla/dom/AnimationEffectReadOnlyBinding.h",
+    "mozilla/dom/KeyframeEffectBinding.h",
     "mozilla/AnimationPropertySegment.h",
     "mozilla/ComputedTiming.h",
     "mozilla/ComputedTimingFunction.h",
     "mozilla/Keyframe.h",
     "mozilla/ServoElementSnapshot.h",
     "mozilla/ServoElementSnapshotTable.h",
     "mozilla/dom/Element.h",
     "mozilla/dom/NameSpaceConstants.h",
@@ -113,16 +114,17 @@ whitelist-types = [
     "mozilla::ComputedTiming",
     "mozilla::ComputedTimingFunction",
     "mozilla::ComputedTimingFunction::BeforeFlag",
     "mozilla::ServoStyleSheet",
     "mozilla::ServoElementSnapshot.*",
     "mozilla::CSSPseudoClassType",
     "mozilla::css::SheetParsingMode",
     "mozilla::css::URLMatchingFunction",
+    "mozilla::dom::IterationCompositeOperation",
     "mozilla::HalfCorner",
     "mozilla::PropertyStyleAnimationValuePair",
     "mozilla::TraversalRestyleBehavior",
     "mozilla::TraversalRootBehavior",
     "mozilla::StyleShapeRadius",
     "mozilla::StyleGrid.*",
     "mozilla::UpdateAnimationsTasks",
     "mozilla::LookAndFeel",
@@ -337,16 +339,17 @@ structs-types = [
     "CSSPseudoElementType",
     "TraversalRestyleBehavior",
     "TraversalRootBehavior",
     "ComputedTimingFunction_BeforeFlag",
     "FontFamilyList",
     "FontFamilyType",
     "FontSizePrefs",
     "GeckoFontMetrics",
+    "IterationCompositeOperation",
     "Keyframe",
     "ServoBundledURI",
     "ServoElementSnapshot",
     "ServoElementSnapshotTable",
     "SheetParsingMode",
     "StyleBasicShape",
     "StyleBasicShapeType",
     "StyleShapeSource",