On the Mac, AX_EVENT_LOAD_COMPLETE should only be fired on the top document.

content/browser/accessibility/browser_accessibility_manager.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/accessibility/browser_accessibility_manager.h"
 
 #include <stddef.h>
 
 #include "base/logging.h"
 #include "build/build_config.h"
@@ skipping 1014 matching lines @@
 }
 
 BrowserAccessibilityDelegate*
     BrowserAccessibilityManager::GetDelegateFromRootManager() {
   BrowserAccessibilityManager* root_manager = GetRootManager();
   if (root_manager)
     return root_manager->delegate();
   return nullptr;
 }
 
+bool BrowserAccessibilityManager::IsRootTree() {
+  return delegate() && delegate()->AccessibilityGetAcceleratedWidget();
+}
+
 ui::AXTreeUpdate
 BrowserAccessibilityManager::SnapshotAXTreeForTesting() {
   std::unique_ptr<
       ui::AXTreeSource<const ui::AXNode*, ui::AXNodeData, ui::AXTreeData>>
       tree_source(tree_->CreateTreeSource());
   ui::AXTreeSerializer<const ui::AXNode*,
                        ui::AXNodeData,
                        ui::AXTreeData> serializer(tree_source.get());
   ui::AXTreeUpdate update;
   serializer.SerializeChanges(tree_->root(), &update);
   return update;
 }
 
 }  // namespace content