--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -529,30 +529,27 @@ ServoStyleSet::ResolveStyleLazily(Elemen
 }
 
 already_AddRefed<ServoStyleContext>
 ServoStyleSet::ResolveInheritingAnonymousBoxStyle(nsIAtom* aPseudoTag,
                                                   ServoStyleContext* aParentContext)
 {
   MOZ_ASSERT(nsCSSAnonBoxes::IsAnonBox(aPseudoTag) &&
              !nsCSSAnonBoxes::IsNonInheritingAnonBox(aPseudoTag));
+  MOZ_ASSERT_IF(aParentContext, !StylistNeedsUpdate());
+
+  UpdateStylistIfNeeded();
+
   RefPtr<ServoStyleContext> style = nullptr;
 
   if (aParentContext) {
     style = aParentContext->GetCachedInheritingAnonBoxStyle(aPseudoTag);
   }
 
   if (!style) {
-    // People like to call into here from random attribute notifications (see
-    // bug 1388234, and bug 1389029).
-    //
-    // We may get a wrong cached style if the stylist needs an update, but we'll
-    // have a whole restyle scheduled anyway.
-    UpdateStylistIfNeeded();
-
     style =
       Servo_ComputedValues_GetForAnonymousBox(aParentContext,
                                               aPseudoTag,
                                               mRawSet.get()).Consume();
     MOZ_ASSERT(style);
     if (aParentContext) {
       aParentContext->SetCachedInheritedAnonBoxStyle(aPseudoTag, style);
     }