JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent"> b
    <div class="child"> a </div>
</div>

CSS

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

.child {
    
    height: 100px;
    width: 100px;
    background-color:yellow;
}

JavaScript

var parent = $('.parent');
    var parentDimensions = parent.height() * parent.width();

    var child = $('.child');
    var childDimensions = child.height() * child.width();

parent.html();
child.html(childDimensions);