css写一个向下箭头

.left{
  position:relative;
  &::after {
    content:'';
    position:absolute;
    right:50%;
    top:100%;
    transform: translateX(50%);
    border-top:20px solid white;
    border-right:20px solid transparent;
    border-left:20px solid transparent;
    border-bottom:20px solid transparent;
  }
}