JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
  <meta http-equiv="Content-Type" content="text/html;charset=utf-8">
  <title>Example</title>
</head>
<body>
  <h1>Example</h1>
    
  <div id="container">
    
  </div>
  <p id="msg">Boo</p>
</body>
</html>

CSS

#container {
    width: 100px;
    border-left: 10px solid black;
    border-right: 10px solid red;
    padding: 4px;
    background-color: #F99;
}

JavaScript

var offsetWidth = document.getElementById('container').offsetWidth;
var clientWidth = document.getElementById('container').clientWidth;
var msg = "offsetWidth: " + offsetWidth + "<br>\n";
msg += "clientWidth: " + clientWidth + "<br>\n";
document.getElementById('msg').innerHTML = msg;
//alert(document.getElementById('msg').innerHTML);