from https://developer.mozilla.org/en-US/docs/Web/CSS/:fullscreen

Making it fullscreen

<script>
// fullscreen stuff
var fullscreenButton = document.getElementById("fullscreen-button");
var fullscreenDiv    = document.getElementById("crtCanvas");
var fullscreenFunc   = fullscreenDiv.requestFullscreen;
if (!fullscreenFunc) {
  ['mozRequestFullScreen',
   'msRequestFullscreen',
   'webkitRequestFullScreen'].forEach(function (req) {
     fullscreenFunc = fullscreenFunc || fullscreenDiv[req];
   });
}
function enterFullscreen() {
  fullscreenFunc.call(fullscreenDiv);
}
fullscreenButton.addEventListener('click', enterFullscreen);
</script>