html { height: 100%; }

body {
  min-height: 100%;
  margin: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  background: #435;
  color: white;
  font-family: sans-serif;
  text-align: center;
}

canvas {
  display: block;
  margin: 0 auto;
  cursor: move;
}