JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">test</div>

CSS

.box {
    border: 1px solid #ddd;
    position: relative;
}
.box:after {
    position: absolute;
    display: block;
    content:'';
    /* Positioning */
    top: 0;
    width: 100%;
    height: 3px;
    left: 0;
    right: 0;
    /* Color */
    background-color: #3F9BD0;
}