body {
  margin: 0;
  padding: 0;
  background: #000;
  color: #9cf;
  font-family: -apple-system, BlinkMacSystemFont, sans-serif;
  height: 100vh;
  width: 100vw;
  overflow: hidden;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
}

#title {
  position: fixed;
  top: 40px;
  left: 0;
  width: 100%;
  text-align: center;
  font-size: 15px;
  letter-spacing: 6px;
  opacity: 0.75;
  z-index: 20;
  pointer-events: none;
}

#startBtn {
  background: #0a3a6a;
  color: #9cf;
  border: 2px solid #4af;
  padding: 22px 40px;
  font-size: 16px;
  letter-spacing: 2px;
  border-radius: 8px;
  min-width: 260px;
  z-index: 20;
  cursor: pointer;
}

video, canvas {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  object-fit: cover;
  z-index: 1;
}

video {
  opacity: 0;
}
3. JS panel