ht: 20px;
  }
  #container::scroll-marker-group {
    display: flex;
    height: 20px;
    background: hotpink;
  }
  #container::column::scroll-marker {
    content: "";
    width: 20px;
    height: 20px;
    margin-right: 5px;
    background: blue;
  }
  #container::column::scroll-marker:focus {
    background: cyan;
  }
</style>
<div id="container">
  <!-- Column #0 -->
  <div tabindex="0" id="c0" style="height:100px;">First</div>

  <!-- Column #1 -->
  <div tabindex="0" id="c1first">line</div>
  <div t