html, body {
  margin: 0;
  padding: 0;
  overflow: hidden;
  width: 100%;
  height: 100%;
  background: #000;
}

a-scene {
  width: 100vw;
  height: 100vh;
}

canvas {
  display: block;
}

#startBtn {
  position: fixed;
  z-index: 9999;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  padding: 14px 22px;
  font-size: 16px;
  border: none;
  border-radius: 999px;
  background: white;
  color: black;
}