<!DOCTYPE html>

<style>
dialog {
  background-color: green;
  height: 50px;
  width: 50px;
  border: none;
  padding: 0;
  margin: 0;

  position: absolute;
  top: 100px;
  left: 100px;
}
</style>

<dialog></dialog>

<script>
document.querySelector('dialog').showModal();
</script>
