--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -7680,17 +7680,17 @@ nsCSSFrameConstructor::ContentAppended(n
 
   if (!isNewShadowTreeContent) {
     // See comment in ContentRangeInserted for why this is necessary.
     if (!GetContentInsertionFrameFor(aContainer) &&
         !aContainer->IsActiveChildrenElement()) {
       // We're punting on frame construction because there's no container frame.
       // The Servo-backed style system handles this case like the lazy frame
       // construction case.
-      if (isNewlyAddedContentForServo) {
+      if (isNewlyAddedContentForServo && aAllowLazyConstruction) {
         LazilyStyleNewChildRange(aFirstNewContent, nullptr);
       }
       return;
     }
 
     if (aAllowLazyConstruction &&
         MaybeConstructLazily(CONTENTAPPEND, aContainer, aFirstNewContent)) {
       if (isNewlyAddedContentForServo) {
@@ -8164,17 +8164,17 @@ nsCSSFrameConstructor::ContentRangeInser
     nsContainerFrame* parentFrame = GetContentInsertionFrameFor(aContainer);
     // The xbl:children element won't have a frame, but default content can have the children as
     // a parent. While its uncommon to change the structure of the default content itself, a label,
     // for example, can be reframed by having its value attribute set or removed.
     if (!parentFrame && !aContainer->IsActiveChildrenElement()) {
       // We're punting on frame construction because there's no container frame.
       // The Servo-backed style system handles this case like the lazy frame
       // construction case.
-      if (isNewlyAddedContentForServo) {
+      if (isNewlyAddedContentForServo && aAllowLazyConstruction) {
         LazilyStyleNewChildRange(aStartChild, aEndChild);
       }
       return;
     }
 
     // Otherwise, we've got parent content. Find its frame.
     NS_ASSERTION(!parentFrame || parentFrame->GetContent() == aContainer ||
                  GetDisplayContentsStyleFor(aContainer), "New XBL code is possibly wrong!");