JSFiddle - React, Tailwind, and code Playground

HTML

<div>
</div>

CSS

div {
    width: 300px;
    height: 100px;
    background-color: red;
}
div:before {
    content: '';
    display: block;
    padding-bottom: 100%;
    background-color: blue;
    width: 100px;
}