JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent">
    <div id="child">
</div>

CSS

#parent {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 300px;
    height: 100px;
    background-color: black;
    border: 1px solid grey;
}
#child {
    height: 100px;
    width: 20px;
    background-color:red;
}