JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <nav id="topnav"></nav>
</div>

CSS

body, html {
    height: 100%;
    margin: 0;
    padding: 0;
    background-color: black;
}
#container {
    position: relative;
    margin: 0 auto;
    width: 400px;
    height: 100%;
    background-color: #666;
}
#topnav {
    width: 100%;
    height: 40px;
    margin: 30px 0;
    background-color: red;
}