JSFiddle - React, Tailwind, and code Playground

HTML

<div class="mybox">
	Le content de box.
</div>

CSS

.mybox {
	position: relative;
	padding: 10px 20px;
	background-color: #EEEEEE;
}

.mybox:after {
	content: '';
	position: absolute;
	bottom: 0px;
	left: 25%;
	width: 50%;
	border-bottom: 1px solid #0000CC;
}