--- a/dom/animation/EffectCompositor.cpp
+++ b/dom/animation/EffectCompositor.cpp
@@ -8,16 +8,17 @@
 
 #include "mozilla/dom/Animation.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/KeyframeEffectReadOnly.h"
 #include "mozilla/AnimationComparator.h"
 #include "mozilla/AnimationPerformanceWarning.h"
 #include "mozilla/AnimationTarget.h"
 #include "mozilla/AnimationUtils.h"
+#include "mozilla/AutoRestore.h"
 #include "mozilla/EffectSet.h"
 #include "mozilla/LayerAnimationInfo.h"
 #include "mozilla/RestyleManager.h"
 #include "mozilla/RestyleManagerInlines.h"
 #include "mozilla/ServoStyleSet.h"
 #include "mozilla/StyleAnimationValue.h"
 #include "mozilla/TypeTraits.h" // For Forward<>
 #include "nsComputedDOMStyle.h" // nsComputedDOMStyle::GetPresShellForContent
@@ -310,17 +311,17 @@ EffectCompositor::PostRestyleForAnimatio
   nsRestyleHint hint = aCascadeLevel == CascadeLevel::Transitions ?
                                         eRestyle_CSSTransitions :
                                         eRestyle_CSSAnimations;
 
   if (mPresContext->StyleSet()->IsServo()) {
     MOZ_ASSERT(NS_IsMainThread(),
                "Restyle request during restyling should be requested only on "
                "the main-thread. e.g. after the parallel traversal");
-    if (ServoStyleSet::IsInServoTraversal()) {
+    if (ServoStyleSet::IsInServoTraversal() || mIsInPreTraverse) {
       MOZ_ASSERT(hint == eRestyle_CSSAnimations ||
                  hint == eRestyle_CSSTransitions);
 
       // We can't call Servo_NoteExplicitHints here since AtomicRefCell does not
       // allow us mutate ElementData of the |aElement| in SequentialTask.
       // Instead we call Servo_NoteExplicitHints for the element in PreTraverse()
       // which will be called right before the second traversal that we do for
       // updating CSS animations.
@@ -955,16 +956,19 @@ EffectCompositor::PreTraverse()
 }
 
 bool
 EffectCompositor::PreTraverseInSubtree(Element* aRoot)
 {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(mPresContext->RestyleManager()->IsServo());
 
+  AutoRestore<bool> guard(mIsInPreTraverse);
+  mIsInPreTraverse = true;
+
   // We need to force flush all throttled animations if there are any
   // non-animation restyles.
   bool flushThrottledRestyles = aRoot && aRoot->HasDirtyDescendantsForServo();
 
   using ElementsToRestyleIterType =
     nsDataHashtable<PseudoElementHashEntry, bool>::Iterator;
   auto getNeededRestyleTarget = [&](const ElementsToRestyleIterType& aIter)
                                 -> NonOwningAnimationTarget {
@@ -1069,16 +1073,19 @@ EffectCompositor::PreTraverse(dom::Eleme
 
   bool found = false;
   if (aPseudoType != CSSPseudoElementType::NotPseudo &&
       aPseudoType != CSSPseudoElementType::before &&
       aPseudoType != CSSPseudoElementType::after) {
     return found;
   }
 
+  AutoRestore<bool> guard(mIsInPreTraverse);
+  mIsInPreTraverse = true;
+
   PseudoElementHashEntry::KeyType key = { aElement, aPseudoType };
 
   // We need to flush all throttled animation restyles too if there are
   // any non-animation restyles.
   Element* elementToRestyle = GetElementToRestyle(aElement, aPseudoType);
   bool flushThrottledRestyles = elementToRestyle &&
                                 elementToRestyle->HasDirtyDescendantsForServo();
 

--- a/dom/animation/EffectCompositor.h
+++ b/dom/animation/EffectCompositor.h
@@ -288,16 +288,18 @@ private:
   // true if a pending animation restyle has also been dispatched. For
   // animations that can be throttled, we will add an entry to the hashtable to
   // indicate that the style rule on the element is out of date but without
   // posting a restyle to update it.
   EnumeratedArray<CascadeLevel, CascadeLevel(kCascadeLevelCount),
                   nsDataHashtable<PseudoElementHashEntry, bool>>
                     mElementsToRestyle;
 
+  bool mIsInPreTraverse = false;
+
   class AnimationStyleRuleProcessor final : public nsIStyleRuleProcessor
   {
   public:
     AnimationStyleRuleProcessor(EffectCompositor* aCompositor,
                                 CascadeLevel aCascadeLevel)
       : mCompositor(aCompositor)
       , mCascadeLevel(aCascadeLevel)
     {