JSFiddle - React, Tailwind, and code Playground

HTML

<style>
/* Remove me to see the correct innerWidth */
@import url('http://fonts.googleapis.com/css?family=Princess+Sofia');

#testcase {
    width: 640px;
    height: 360px;
    background: grey;
}
</style>
<div id="testcase"></div>

JavaScript

console.log('clientWidth', document.getElementById('testcase').clientWidth );