JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div {
    width: 25%;
    border: 5px solid #ccc;
}

JavaScript

var div = document.getElementsByTagName('div')[0];
div.style.height = div.clientWidth + 'px';

window.onresize = function() {
    div.style.height = div.clientWidth + 'px';
}