--- a/dom/animation/KeyframeEffect.cpp
+++ b/dom/animation/KeyframeEffect.cpp
@@ -15,17 +15,16 @@
 #include "mozilla/KeyframeUtils.h"
 #include "mozilla/StyleAnimationValue.h"
 #include "Layers.h" // For Layer
 #include "nsComputedDOMStyle.h" // nsComputedDOMStyle::GetStyleContextForElement
 #include "nsCSSPropertySet.h"
 #include "nsCSSProps.h" // For nsCSSProps::PropHasFlags
 #include "nsCSSPseudoElements.h" // For CSSPseudoElementType
 #include "nsDOMMutationObserver.h" // For nsAutoAnimationMutationBatch
-#include <algorithm> // For std::max
 
 namespace mozilla {
 
 // Helper functions for generating a ComputedTimingProperties dictionary
 static void
 GetComputedTimingDictionary(const ComputedTiming& aComputedTiming,
                             const Nullable<TimeDuration>& aLocalTime,
                             const TimingParams& aTiming,
@@ -241,17 +240,20 @@ KeyframeEffectReadOnly::GetComputedTimin
     MOZ_ASSERT(aTiming.mDuration.ref() >= zeroDuration,
                "Iteration duration should be positive");
     result.mDuration = aTiming.mDuration.ref();
   }
 
   result.mIterations = IsNaN(aTiming.mIterations) || aTiming.mIterations < 0.0f ?
                        1.0f :
                        aTiming.mIterations;
-  result.mIterationStart = std::max(aTiming.mIterationStart, 0.0);
+  MOZ_ASSERT(aTiming.mIterationStart >= 0.0,
+             "mIterationStart should be nonnegative, as ensured by "
+             "ValidateIterationStart");
+  result.mIterationStart = aTiming.mIterationStart;
 
   result.mActiveDuration = ActiveDuration(result.mDuration, result.mIterations);
   result.mEndTime = aTiming.mDelay + result.mActiveDuration +
                     aTiming.mEndDelay;
   result.mFill = aTiming.mFill == dom::FillMode::Auto ?
                  dom::FillMode::None :
                  aTiming.mFill;