如何在加载网站上添加移动线？

@media screen and (max-width: 768px) {
  /* 在小屏幕设备上显示移动线 */
  body::before {
    content: "";
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 2px;
    background-color: #000;
    z-index: 9999;
  }
}

<script>
  window.addEventListener('DOMContentLoaded', function() {
    var mobileLine = document.createElement('div');
    mobileLine.style.position = 'fixed';
    mobileLine.style.top = '0';
    mobileLine.style.left = '0';
    mobileLine.style.width = '100%';
    mobileLine.style.height = '2px';
    mobileLine.style.backgroundColor = '#000';
    mobileLine.style.zIndex = '9999';
    document.body.insertBefore(mobileLine, document.body.firstChild);
  });
</script>