.kk:hover:after{
left: 0%;
width: 100%;
}

<div id="" class="flexs">
<div class="kk">123</div>
<div class="kk">456</div>
<div class="kk">789</div>
<div class="kk">012</div>
</div>

css-hover-after鼠标移入展示下划线