JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header"></div>
<div class="main-content"></div>

CSS

body {
    display: inline-block;
}

.header {
    width: 100%;
    height: 40px;
    background-color: #333;
}

.main-content {
    background-color: #888;
    height: 200px;
    width: 600px;
}