Dialog

<dialog></dialog>

<dialog>
  <form method="dialog">
    …
    <button value="cancel">Cancel</button>
    <button value="confirm">Confirm</button>
  </form>
</dialog>

document.querySelector('dialog').showModal();
document.querySelector('dialog').close();

<dialog id="dialog">
  <p>Hi, I'm a dialog.</p>
  <button commandfor="dialog" command="close">Ok</button>
</dialog>

<button commandfor="dialog" command="showModal">Open Dialog</button>