bootstrap 5，带指示器的标签

  .arrow {
    position:absolute;
    top: -26px;
    left: 0;
    right:0;
    text-align: center;
  }
  .arrow::after {
    content: '';
    width: 0; 
    height: 0; 
    border-right: 10px solid transparent;
    border-left: 10px solid transparent;
    border-bottom: 8px solid black;
  }
  
  .black-bg {
    background-color:#000;
    color: #fff;
    width:100px;
    height:50px;
    position: relative;
  }

<div class="black-bg">
  <div class="arrow"></div>
</div>