html,
body {
  margin: 0;
  padding: 0;
  width: 100%;
  height: 100%;
  overflow: hidden;
  display: grid;
  place-items: center;
  background: #000;
}

canvas {
  display: block;
  /* p5 writes the fixed 720 × 960 display size inline. Override only that
     display size so the unchanged design-coordinate canvas is contained. */
  width: min(100vw, 75vh) !important;
  height: min(100vh, 133.333333vw) !important;
}