er::scroll-marker-group {
    border: 3px solid black;
    padding: 5px;
    display: block;
    height: 40px;
  }

  #scroller div::scroll-marker {
    content: "";
    width: 10px;
    height: 10px;
    background-color: blue;
    border-radius: 100%;
    display: inline-block;
  }

  #scroller #first::scroll-marker {
    background-color: purple;
    margin-right: 4px;
  }
</style>
<div>TEST BLOCK BEFORE</div>
<div id="scroller">
  <div id="first"></div>
  <div></div>
</div>
<div>TEST BLOCK AFTER</div>