body{margin:0;background:#000;display:flex;justify-content:center;align-items:center;height:100vh;font-family:sans-serif;overflow:hidden}#fullscreenBtn{position:absolute;top:50%;left:50%;transform:translate(-50%,-50%);background:rgba(255,255,255,0.15);color:white;border:1px solid white;padding:15px 30px;border-radius:8px;cursor:pointer;opacity:.7;z-index:999;font-size:18px;opacity:.2}