--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -4385,16 +4385,19 @@ NoteDirtyElement(Element* aElement, uint
       // make sure the bit we propagated is also registered on the root.
       doc->SetServoRestyleRoot(existingRoot, existingBits | aBit);
   } else {
     // We reached the root without crossing the pre-existing restyle root. We
     // now need to find the nearest common ancestor, so climb up from the
     // existing root, extending bits along the way.
     Element* rootParent = existingRoot->GetFlattenedTreeParentElementForStyle();
     if (Element* commonAncestor = PropagateBits(rootParent, existingBits, aElement)) {
+      MOZ_ASSERT(commonAncestor == aElement ||
+                 commonAncestor == nsContentUtils::GetCommonFlattenedTreeAncestorForStyle(aElement, rootParent));
+
       // We found a common ancestor. Make that the new style root, and clear the
       // bits between the new style root and the document root.
       doc->SetServoRestyleRoot(commonAncestor, existingBits | aBit);
       Element* curr = commonAncestor;
       while ((curr = curr->GetFlattenedTreeParentElementForStyle())) {
         MOZ_ASSERT(curr->HasFlag(aBit));
         curr->UnsetFlags(aBit);
       }