JSFiddle - React, Tailwind, and code Playground

HTML

<ul style="margin:0px;padding:0px;list-style:none">
    <li class="box"><img src="http://i00.i.aliimg.com/wsphoto/v1/502581864_2/New-Cartoon-Vinyl-Diy-Platform-Art-Toy-Used-In-Diy-For-Kids-Painting-Sample.jpg_50x50.jpg"/></li>
</ul>
<br/>
<div class="box" style="width:200px;height:200px;background:red">
   Test
</div>

CSS

.box { position: relative; display:inline-block;}
.box:after {
    position: absolute;
    width: 100%;
    height: 10px;
    background: green;
    content: '';
    bottom:-10px;
    left:0;
}