css 使用伪类在div底部画一条线

<div class="div></div>

.div {
     position: relative;
 }
.div::after {
     content: ''; // 必须
     position: absolute;
     left: 30px;
     bottom: 0;
     right: 0;
     width: 720px;
     height: 1PX;
     background-color: #dcdcdc;
 }