JSFiddle - React, Tailwind, and code Playground

HTML

<div class="red">

  <div class="lightblue">
  </div>
  
</div>

CSS

body {margin:0;}
.red {
  width:100%;
  height:500px;
  background-color:red;
  position:relative;
}

JavaScript

var div = document.querySelector(".lightblue");
div.style {
	background-color:lightblue;
  width:100px;
  height:100px;
}