--- a/dom/animation/Animation.cpp
+++ b/dom/animation/Animation.cpp
@@ -791,47 +791,38 @@ Animation::ComposeStyle(RefPtr<AnimValue
   //     that case the compositor might have been ahead of the main thread so we
   //     should use the current wallclock time to ensure the animation doesn't
   //     temporarily jump backwards.
   //
   // To address each of these cases we temporarily tweak the hold time
   // immediately before updating the style rule and then restore it immediately
   // afterwards. This is purely to prevent visual flicker. Other behavior
   // such as dispatching events continues to rely on the regular timeline time.
-  bool updatedHoldTime = false;
   AnimationPlayState playState = PlayState();
   {
     AutoRestore<Nullable<TimeDuration>> restoreHoldTime(mHoldTime);
 
     if (playState == AnimationPlayState::Pending &&
         mHoldTime.IsNull() &&
         !mStartTime.IsNull()) {
       Nullable<TimeDuration> timeToUse = mPendingReadyTime;
       if (timeToUse.IsNull() &&
           mTimeline &&
           mTimeline->TracksWallclockTime()) {
         timeToUse = mTimeline->ToTimelineTime(TimeStamp::Now());
       }
       if (!timeToUse.IsNull()) {
         mHoldTime.SetValue((timeToUse.Value() - mStartTime.Value())
                             .MultDouble(mPlaybackRate));
-        // Push the change down to the effect
-        UpdateEffect();
-        updatedHoldTime = true;
       }
     }
 
     mEffect->ComposeStyle(aStyleRule, aSetProperties);
   }
 
-  // Now that the hold time has been restored, update the effect
-  if (updatedHoldTime) {
-    UpdateEffect();
-  }
-
   MOZ_ASSERT(playState == PlayState(),
              "Play state should not change during the course of compositing");
   mFinishedAtLastComposeStyle = (playState == AnimationPlayState::Finished);
 }
 
 void
 Animation::NotifyEffectTimingUpdated()
 {

--- a/dom/animation/EffectCompositor.cpp
+++ b/dom/animation/EffectCompositor.cpp
@@ -560,16 +560,19 @@ EffectCompositor::ComposeAnimationRule(d
   // As a result, we iterate from last animation to first and, if a
   // property has already been set, we don't change it.
   nsCSSPropertySet properties;
 
   for (KeyframeEffectReadOnly* effect : Reversed(sortedEffectList)) {
     effect->GetAnimation()->ComposeStyle(animationRule, properties);
   }
 
+  MOZ_ASSERT(effects == EffectSet::GetEffectSet(aElement, aPseudoType),
+             "EffectSet should not change while composing style");
+
   effects->UpdateAnimationRuleRefreshTime(aCascadeLevel, aRefreshTime);
 }
 
 /* static */ void
 EffectCompositor::GetOverriddenProperties(nsStyleContext* aStyleContext,
                                           EffectSet& aEffectSet,
                                           nsCSSPropertySet&
                                             aPropertiesOverridden)