body { margin: 0; overflow: hidden; }
canvas { display: block; }
#instructions {
  position: absolute;
  top: 50%; left: 50%;
  transform: translate(-50%, -50%);
  color: white;
  font-family: sans-serif;
  font-size: 24px;
  text-align: center;
  cursor: pointer;
  background: rgba(0,0,0,0.5);
  padding: 20px 40px;
  border-radius: 8px;
  user-select: none;
}