JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">
        <div class="sub-box"></div>
    </div>

CSS

.box {
    width: 300px;
    height: 200px;
    background-color: red;
}

.sub-box {
    width: 200px;
    height: 100px;
    background-color: black;

}

JavaScript

var element = document.getElementsByClassName('box');
var stylesOfElement = window.getComputedStyle(element[0]);

console.log(element);
console.log(stylesOfElement);