伪类before和after

 .one {
        font-size: 50px;
        color: red;
      }

      .one::before {
        /*content必须填写，有没有值无所谓*/
        content: "";
        color: green;
      }

      .one::after{
        content: "after";
        color: purple;
      }

 

伪类

清除浮动

 .clearfix::before,
      .clearfix::after {
        content: "";
        clear: both;
        display: block;
      }