JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mainContent">
    Look ma, it's centered!
</div>

CSS

#mainContent {
    width: 500px;
    height: 200px;
    position:fixed;
    left:50%;
    top:50%;
    margin:-100px 0 0 -250px;
    border: 1px solid #D8D8D8;
    border-radius: 5px;
    background: #E6E6E6;
}