JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
    
 <div class="box"></div>   
</div>

CSS

.main {
    height:300px;
    border:1px red solid;
    position:relative;
}
.box {
    width:40px;
    height:40px;
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    margin: auto;
    background:red;
}