原有元素要添加 position: relative; ::before伪元素一点要加z-index属性保证原有元素处于后加元素上层，否则会导致原有元素上点击等事件触发不了
<div class="top border-line">
</div>.top {
position: relative;
}
.border-line::before {
z-index: -999;content: '';position: absolute;top: 0;left: 0;border-bottom: 1px solid #000;width: 200%;height: 200%;transform-origin: 0 0;transform: scale(0.5, 0.5);box-sizing: border-box;
}