CSS弯曲箭头画法

<div class="box">
  <div class="aa"></div>
</div>
<style>
  .box {
    background-color: blue;
  }

  .aa {
    width: 20px;
    height: 40px;
    background-color: red;
    border-bottom: 3px solid aqua;
    border-radius: 0 0 0 20px;
  }
</style>