JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box">
    <div class="box2">
    </div>
</div>

CSS

body{margin:0;}

#box{background:#000;
    height:200px;
    width:500px;
}

.box2{background:#AAA;
    height:150px;
    width:300px;
    margin:0 auto;}