--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -161,16 +161,23 @@ nsIContent::GetFlattenedTreeParentNodeIn
     MOZ_ASSERT(!parentNode || parentNode == OwnerDoc());
     return parentNode;
   }
   nsIContent* parent = parentNode->AsContent();
 
   if (aType == eForStyle &&
       IsRootOfNativeAnonymousSubtree() &&
       OwnerDoc()->GetRootElement() == parent) {
+    // First, check if this is generated ::before/::after content for the root.
+    // If it is, we know what to do.
+    if (IsGeneratedContentContainerForBefore() ||
+        IsGeneratedContentContainerForAfter()) {
+      return parent;
+    }
+
     // When getting the flattened tree parent for style, we return null
     // for any "document level" native anonymous content subtree root.
     // This is NAC generated by an ancestor frame of the document element's
     // primary frame, and includes scrollbar elements created by the root
     // scroll frame, and the "custom content container" and accessible caret
     // generated by the nsCanvasFrame.  We distinguish document level NAC
     // from NAC generated by the root element's primary frame below.
     nsIFrame* parentFrame = parent->GetPrimaryFrame();