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

<button commandfor="dialog" command="show-modal">Open Dialog</button> 

html {
  block-size: 100%;
  color-scheme: dark light;
}

body {
  min-block-size: 100%;
  font-family: system-ui, sans-serif;
  
  display: grid;
  place-content: center;
  margin: 0;
}