<button id="showMyDialog">Show dialog</button>
<dialog id="mydialog">
  Hello World<br/>
  <button id="dismiss">Dismiss</button>
</dialog>

showButton = document.getElementById("showMyDialog");
showButton.addEventListener("click", showMyDialog);
dismissButton = document.getElementById("dismiss");
dismissButton.addEventListener("click", closeMyDialog);
function showMyDialog() {
  mydialog = document.getElementById("mydialog");
  mydialog.showModal();
}
function closeMyDialog() {
  mydialog = document.getElementById("mydialog");
  mydialog.close();
}