--- a/dom/animation/KeyframeEffectReadOnly.cpp
+++ b/dom/animation/KeyframeEffectReadOnly.cpp
@@ -648,20 +648,17 @@ KeyframeEffectParamsFromUnion(const Opti
                                        result.mPacedProperty,
                                        aInvalidPacedProperty,
                                        aCallerType,
                                        aRv);
     // Ignore iterationComposite if the Web Animations API is not enabled,
     // then the default value 'Replace' will be used.
     if (AnimationUtils::IsCoreAPIEnabledForCaller(aCallerType)) {
       result.mIterationComposite = options.mIterationComposite;
-      // FIXME: Bug 1311620: We don't support additive animation yet.
-      if (options.mComposite != dom::CompositeOperation::Add) {
-        result.mComposite = options.mComposite;
-      }
+      result.mComposite = options.mComposite;
     }
   }
   return result;
 }
 
 /* static */ Maybe<OwningAnimationTarget>
 KeyframeEffectReadOnly::ConvertTarget(
   const Nullable<ElementOrCSSPseudoElement>& aTarget)

deleted file mode 100644
--- a/testing/web-platform/meta/web-animations/animation-model/combining-effects/effect-composition.html.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[effect-composition.html]
-  type: testharness
-  [add specified on a keyframe overrides the composite mode of the effect]
-    expected: FAIL
-