JSFiddle - React, Tailwind, and code Playground

HTML

<div id="centerme">
    Hello, world!
</div>

CSS

#centerme
{
    position: absolute;
    left: 50%;
    top: 50%;
    
    /* You must set a size manually */
    width: 100px;
    height: 50px;
    
    /* Set negative margins equal to half the size */
    margin-left: -50px;
    margin-top: -25px;
    
    background-color: cornflowerblue;
}