--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -290,18 +290,17 @@ ServoStyleSet::PrepareAndTraverseSubtree
                                          TraversalRestyleBehavior
                                            aRestyleBehavior)
 {
   // Get the Document's root element to ensure that the cache is valid before
   // calling into the (potentially-parallel) Servo traversal, where a cache hit
   // is necessary to avoid a data race when updating the cache.
   mozilla::Unused << aRoot->OwnerDoc()->GetRootElement();
 
-  MOZ_ASSERT(!sInServoTraversal);
-  sInServoTraversal = true;
+  AutoSetInServoTraversal guard(this);
 
   bool isInitial = !aRoot->HasServoData();
   bool forReconstruct =
     aRestyleBehavior == TraversalRestyleBehavior::ForReconstruct;
   bool postTraversalRequired =
     Servo_TraverseSubtree(aRoot, mRawSet.get(), aRootBehavior, aRestyleBehavior);
   MOZ_ASSERT_IF(isInitial || forReconstruct, !postTraversalRequired);
 
@@ -333,17 +332,16 @@ ServoStyleSet::PrepareAndTraverseSubtree
         MOZ_ASSERT(!postTraversalRequired);
         ServoRestyleManager::ClearRestyleStateFromSubtree(root);
       } else {
         postTraversalRequired = true;
       }
     }
   }
 
-  sInServoTraversal = false;
   return postTraversalRequired;
 }
 
 already_AddRefed<nsStyleContext>
 ServoStyleSet::ResolveStyleFor(Element* aElement,
                                nsStyleContext* aParentContext,
                                LazyComputeBehavior aMayCompute,
                                TreeMatchContext& aTreeMatchContext)
@@ -978,18 +976,18 @@ ServoStyleSet::ClearNonInheritingStyleCo
     ptr = nullptr;
   }
 }
 
 already_AddRefed<ServoComputedValues>
 ServoStyleSet::ResolveStyleLazily(Element* aElement, nsIAtom* aPseudoTag)
 {
   mPresContext->EffectCompositor()->PreTraverse(aElement, aPseudoTag);
-  MOZ_ASSERT(!sInServoTraversal);
-  sInServoTraversal = true;
+
+  AutoSetInServoTraversal guard(this);
 
   /**
    * NB: This is needed because we process animations and transitions on the
    * pseudo-elements themselves, not on the parent's EagerPseudoStyles.
    *
    * That means that that style doesn't account for animations, and we can't do
    * that easily from the traversal without doing wasted work.
    *
@@ -1018,18 +1016,16 @@ ServoStyleSet::ResolveStyleLazily(Elemen
 
   if (mPresContext->EffectCompositor()->PreTraverse(aElement, aPseudoTag)) {
     computedValues =
       Servo_ResolveStyleLazily(elementForStyleResolution,
                                pseudoTagForStyleResolution,
                                mRawSet.get()).Consume();
   }
 
-  sInServoTraversal = false;
-
   return computedValues.forget();
 }
 
 bool
 ServoStyleSet::AppendFontFaceRules(nsTArray<nsFontFaceRuleContainer>& aArray)
 {
   Servo_StyleSet_GetFontFaceRules(mRawSet.get(), &aArray);
   return true;
@@ -1105,9 +1101,9 @@ ServoStyleSet::RemoveSheetOfType(SheetTy
       uint32_t uniqueID = mEntries[aType][i].uniqueID;
       mEntries[aType].RemoveElementAt(i);
       return uniqueID;
     }
   }
   return 0;
 }
 
-bool ServoStyleSet::sInServoTraversal = false;
+ServoStyleSet* ServoStyleSet::sInServoTraversal = nullptr;

--- a/layout/style/ServoStyleSet.h
+++ b/layout/style/ServoStyleSet.h
@@ -80,16 +80,21 @@ public:
     // maintain this static boolean. However, the danger is that those callers
     // are generally unprepared to deal with non-Servo-but-also-non-main-thread
     // callers, and are likely to take the main-thread codepath if this function
     // returns false. So we assert against other non-main-thread callers here.
     MOZ_ASSERT(sInServoTraversal || NS_IsMainThread());
     return sInServoTraversal;
   }
 
+  static ServoStyleSet* Current()
+  {
+    return sInServoTraversal;
+  }
+
   ServoStyleSet();
   ~ServoStyleSet();
 
   void Init(nsPresContext* aPresContext);
   void BeginShutdown();
   void Shutdown();
 
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
@@ -304,16 +309,33 @@ public:
   ResolveForDeclarations(ServoComputedValuesBorrowedOrNull aParentOrNull,
                          RawServoDeclarationBlockBorrowed aDeclarations);
 
   already_AddRefed<RawServoAnimationValue>
   ComputeAnimationValue(RawServoDeclarationBlock* aDeclaration,
                         const ServoComputedValuesWithParent& aComputedValues);
 
 private:
+  class MOZ_STACK_CLASS AutoSetInServoTraversal
+  {
+  public:
+    explicit AutoSetInServoTraversal(ServoStyleSet* aSet)
+    {
+      MOZ_ASSERT(!sInServoTraversal);
+      MOZ_ASSERT(aSet);
+      sInServoTraversal = aSet;
+    }
+
+    ~AutoSetInServoTraversal()
+    {
+      MOZ_ASSERT(sInServoTraversal);
+      sInServoTraversal = nullptr;
+    }
+  };
+
   already_AddRefed<nsStyleContext> GetContext(already_AddRefed<ServoComputedValues>,
                                               nsStyleContext* aParentContext,
                                               nsIAtom* aPseudoTag,
                                               CSSPseudoElementType aPseudoType,
                                               dom::Element* aElementForAnimation);
 
   already_AddRefed<nsStyleContext> GetContext(nsIContent* aContent,
                                               nsStyleContext* aParentContext,
@@ -394,14 +416,14 @@ private:
   bool mAuthorStyleDisabled;
 
   // Stores pointers to our cached style contexts for non-inheriting anonymous
   // boxes.
   EnumeratedArray<nsCSSAnonBoxes::NonInheriting,
                   nsCSSAnonBoxes::NonInheriting::_Count,
                   RefPtr<nsStyleContext>> mNonInheritingStyleContexts;
 
-  static bool sInServoTraversal;
+  static ServoStyleSet* sInServoTraversal;
 };
 
 } // namespace mozilla
 
 #endif // mozilla_ServoStyleSet_h