JSFiddle - React, Tailwind, and code Playground

HTML

<div id="header"></div>

CSS

#header {
    height: 100px;
    width: 300px;
    background: silver;
    position: relative;
}

#header:after {
    display: block;
    content: '';
    height: 20px;
    width: 300px;
    background: green;
    position: absolute;
    top: 40px;
}