--- a/dom/animation/KeyframeEffect.cpp
+++ b/dom/animation/KeyframeEffect.cpp
@@ -5,16 +5,17 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/dom/KeyframeEffect.h"
 #include "mozilla/dom/KeyframeEffectBinding.h"
 #include "mozilla/FloatingPoint.h"
 #include "AnimationCommon.h"
 #include "nsCSSPropertySet.h"
 #include "nsCSSProps.h" // For nsCSSProps::PropHasFlags
+#include "nsStyleUtil.h"
 
 namespace mozilla {
 
 void
 ComputedTimingFunction::Init(const nsTimingFunction &aFunction)
 {
   mType = aFunction.mType;
   if (nsTimingFunction::IsSplineType(mType)) {
@@ -48,16 +49,38 @@ ComputedTimingFunction::GetValue(double 
     // up to zero, so we can't do that.  And it's not clear the spec
     // really meant it.
     return 1.0 - StepEnd(mSteps, 1.0 - aPortion);
   }
   MOZ_ASSERT(mType == nsTimingFunction::Type::StepEnd, "bad type");
   return StepEnd(mSteps, aPortion);
 }
 
+void
+ComputedTimingFunction::AppendToString(nsAString& aResult) const
+{
+  switch (mType) {
+    case nsTimingFunction::Type::CubicBezier:
+      nsStyleUtil::AppendCubicBezierTimingFunction(mTimingFunction.X1(),
+                                                   mTimingFunction.Y1(),
+                                                   mTimingFunction.X2(),
+                                                   mTimingFunction.Y2(),
+                                                   aResult);
+      break;
+    case nsTimingFunction::Type::StepStart:
+    case nsTimingFunction::Type::StepEnd:
+      nsStyleUtil::AppendStepsTimingFunction(mType, mSteps, mStepSyntax,
+                                             aResult);
+      break;
+    default:
+      nsStyleUtil::AppendCubicBezierKeywordTimingFunction(mType, aResult);
+      break;
+  }
+}
+
 // In the Web Animations model, the iteration progress can be outside the range
 // [0.0, 1.0] but it shouldn't be Infinity.
 const double ComputedTiming::kNullProgress = PositiveInfinity<double>();
 
 namespace dom {
 
 NS_IMPL_CYCLE_COLLECTION_INHERITED(KeyframeEffectReadOnly,
                                    AnimationEffectReadOnly,

--- a/dom/animation/KeyframeEffect.h
+++ b/dom/animation/KeyframeEffect.h
@@ -125,16 +125,17 @@ public:
            (HasSpline() ?
             mTimingFunction == aOther.mTimingFunction :
             (mSteps == aOther.mSteps &&
              mStepSyntax == aOther.mStepSyntax));
   }
   bool operator!=(const ComputedTimingFunction& aOther) const {
     return !(*this == aOther);
   }
+  void AppendToString(nsAString& aResult) const;
 
 private:
   Type mType;
   nsSMILKeySpline mTimingFunction;
   uint32_t mSteps;
   StepSyntax mStepSyntax;
 };