JSFiddle - React, Tailwind, and code Playground

HTML

<div id='output'>
  Processing... :p
</div>
<div>
  Sooo... How do I get the width and height of whats-my-dims1?
</div>
<div id='some-hidden-div'>
  <div class='whats-my-dims' id='whats-my-dims1'></div>
</div>
<div class='whats-my-dims' id='whats-my-dims2'></div>

CSS

#some-hidden-div{
  display: none;
}
.whats-my-dims{
  display:block;
  width: 69px;
  height: 42px;
  background-color: #f00;
}

JavaScript

var o = document.getElementById('output');
var wmd1 = document.getElementById('whats-my-dims1');
var wmd2 = document.getElementById('whats-my-dims2');
o.innerHTML = 'wmd1: "' + window.getComputedStyle(wmd1).getPropertyValue("width") + '", "' + window.getComputedStyle(wmd1).getPropertyValue("height") + '", wmd2: "' + window.getComputedStyle(wmd2).getPropertyValue("width") + '", "' + window.getComputedStyle(wmd2).getPropertyValue("height") + '"';