JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outer">
    <div id="inner"></div>
</div>

CSS

#outer {
    background: #33f;
    width: 500px;
    height: 300px;
}

#inner {
    background: #77f;
    width: 50%;
    height: 70%;
}

JavaScript

var element = document.getElementById("inner");

element.innerHTML = element.clientWidth  + "px" + "<br>" 
                  + element.clientHeight + "px";