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");
var divStyle = div.style.cssText;

var style = [
	"background-color:blue;",
    "width:100px;",
    "height:100px;"
];


for(var i = 0; i < style.length; i++) {
	div.style.cssText += style[i];
}