JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mydiv"></div>
adsf<br>adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>adsf<br>adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>
adsf<br>

CSS

#mydiv {
    top: 50%;
    left: 50%;
    width:30em;
    height:18em;
    margin-top: -9em; /*set to a negative number 1/2 of your height*/
    margin-left: -15em; /*set to a negative number 1/2 of your width*/
    border: 1px solid #ccc;
    background-color: #f3f3f3;
    position:fixed;
}