chromium/content/test/data/accessibility/aom/aom-modal-dialog.html

<!DOCTYPE html>
<!--
@BLINK-ALLOW:modal*
@WIN-ALLOW:IA2_STATE_MODAL
@WIN-ALLOW:container*
@UIA-WIN-ALLOW:Window.IsModal*
@UIA-WIN-ALLOW:LocalizedControlType*
@AURALINUX-ALLOW:modal*

TODO: For Mac and Android, we need to prune the content outside modal dialog.
http://crbug.com/1165298
-->
<html>
<body>
  <div>
   Content outside modal dialog.
   <button>Button outside modal dialog.</button>
  </div>
  <script>
    var button_node = new AccessibleNode();
    button_node.role = "button";
    button_node.label = "Button inside modal dialog.";

    var dialog_node = new AccessibleNode();
    dialog_node.role = "dialog";
    dialog_node.modal = "true";
    dialog_node.label = "Modal dialog."
    document.body.accessibleNode.appendChild(dialog_node);
    dialog_node.appendChild(button_node);
  </script>
</body>
</html>