伪元素：

    1. .clearfix {
    2.   &::before {
    3.     content: '';
    4.     display:table;
    5.   }
    6.   &::after {
    7.     visibility: hidden;
    8.     display: block;
    9.     font-size: 0;
    10.     content: '';
    11.     clear: both;
    12.     height: 0;
    13.   }
    14. }