var result = window.showModalDialog('/confirm.html', {}, 'dialogWidth:300px;');

<dialog id="dlg">
  <form method="dialog">
    <p>Proceed?</p>
    <button value="yes">Yes</button>
    <button value="no">No</button>
  </form>
</dialog>
<script>
  async function ask() {
    const dlg = document.getElementById('dlg');
    dlg.showModal();
    const choice = await new Promise(r => dlg.addEventListener('close', () => r(dlg.returnValue), {once:true}));
    return choice;
  }
</script>