--- a/dom/html/HTMLSlotElement.cpp
+++ b/dom/html/HTMLSlotElement.cpp
@@ -125,37 +125,35 @@ FlattenAssignedNodes(HTMLSlotElement* aS
   if (!aSlot->GetContainingShadow()) {
     return;
   }
 
   const nsTArray<RefPtr<nsINode>>& assignedNodes = aSlot->AssignedNodes();
 
   // If assignedNodes is empty, use children of slot as fallback content.
   if (assignedNodes.IsEmpty()) {
-    for (nsIContent* child = aSlot->AsContent()->GetFirstChild();
+    for (nsIContent* child = aSlot->GetFirstChild();
          child;
          child = child->GetNextSibling()) {
       if (!child->IsSlotable()) {
         continue;
       }
 
-      if (child->IsHTMLElement(nsGkAtoms::slot)) {
-        FlattenAssignedNodes(HTMLSlotElement::FromNode(child), aNodes);
+      if (auto* slot = HTMLSlotElement::FromNode(child)) {
+        FlattenAssignedNodes(slot, aNodes);
       } else {
         aNodes.AppendElement(child);
       }
     }
     return;
   }
 
-  for (uint32_t i = 0; i < assignedNodes.Length(); i++) {
-    nsINode* assignedNode = assignedNodes[i];
-    if (assignedNode->IsHTMLElement(nsGkAtoms::slot)) {
-      FlattenAssignedNodes(
-        HTMLSlotElement::FromNode(assignedNode->AsContent()), aNodes);
+  for (const RefPtr<nsINode>& assignedNode : assignedNodes) {
+    if (auto* slot = HTMLSlotElement::FromNode(assignedNode)) {
+      FlattenAssignedNodes(slot, aNodes);
     } else {
       aNodes.AppendElement(assignedNode);
     }
   }
 }
 
 void
 HTMLSlotElement::AssignedNodes(const AssignedNodesOptions& aOptions,