css 箭头

.toTop{
   2.5rem;
  height: 2.5rem;
  background-color: rgba(228,228,228,.6);
  position: fixed;
  bottom: 5rem;
  text-align: center;
  line-height: 2.5rem;
  right: 2rem;
  display: -webkit-flex;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 3px;
  i{
     2rem;
    height: 2rem;
    position: relative;
    &:after{
      content: "";
       35%;
      height: 35%;
      display: inline-block;
      border-top: .1rem solid #333;
      border-left: .1rem solid #333;
      overflow: hidden;
      margin: .6rem 1rem;
      transform: rotate(45deg);
      transform-origin: 0 0;
    }
  }
}

<div class="toTop"> <i></i> </div>