[spartanatreyu]

I didn't need any JS here:

  <dialog open>
    <p>Greetings, one and all!</p>
    <form method="dialog">
      <button>OK</button>
    </form>
  </dialog>

[robertoandred]

And if you don’t want it open when you load the page?

[spartanatreyu]

get rid of the open attribute