/* Basic page reset */
* { margin:0; padding:0; box-sizing:border-box; }
html, body {
  width:100%; height:100%;
  background:#000;
  display:flex; justify-content:center; align-items:center;
}

/* Canvas takes the size we gave it */
canvas#game {
  background:#000; /* black space */
  border:1px solid #555;
}