JSFiddle - React, Tailwind, and code Playground

HTML

<h2>The blue box is centered in its container.</h2>

<div class='container'>
    <div class='centered_box'>
    </div>
</div>

CSS

.centered_box {
    width: 300px;
    height: 200px;
    background: blue;
    color: #fff;
}

.container{
    display: flex;
    flex-direction: column;
    justify-content: space-around;
    align-items: center;
    width: 500px;
    height: 500px;
    border: 5px solid #000;
}