JSFiddle - React, Tailwind, and code Playground

HTML

<div>aaa</div>

CSS

div {
    display: block;
    height: 200px;
    width: 500px;
    border: solid 1px #CCC;
    border-bottom: none;
    position: relative;
}
div:after {
    content:"";
    background: -webkit-linear-gradient(left, rgba(92, 7, 52, 1) 0%, rgba(134, 29, 84, 1) 12%, rgba(255, 93, 177, 1) 47%, rgba(83, 0, 30, 1) 100%);
    display: block;
    height:10px;
    width: 500px;
    position: absolute;
    bottom: 0;
}