diff --git a/third_party/blink/renderer/core/dom/container_node.cc b/third_party/blink/renderer/core/dom/container_node.cc
index bb8769ce97f61..3105fa6ba8c03 100644
--- a/third_party/blink/renderer/core/dom/container_node.cc
+++ b/third_party/blink/renderer/core/dom/container_node.cc
@@ -279,6 +279,19 @@ bool ContainerNode::EnsurePreInsertionValidity(
     // Step 3:
     if (!CheckReferenceChildParent(*this, next, old_child, exception_state))
       return false;
+    // When called with a node vector (multi-argument append/prepend/etc.),
+    // ConvertNodeUnionsIntoNodes() already removed each node from its old
+    // parent, which may have run synchronous event handlers (blur, iframe
+    // unload). Re-validate that none of the nodes were reparented during
+    // those handlers; otherwise InsertNodeVector() would link a node into a
+    // second child list without unlinking it from the first.
+    if (new_children && new_children->size() != 1u &&
+        RuntimeEnabledFeatures::
+            RecheckParentDuringNodeVectorInsertionEnabled() &&
+        !RecheckNodeInsertionStructuralPrereq(*new_children, next,
+                                              exception_state)) {
+      return false;
+    }
     // Step 4-6.
     return document->CanAcceptChild(new_child, new_children, next, old_child,
                                     exception_state);