JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
  <head>
    <style>
        html {
            background-color: red;
        }
    </style>
    <meta charset="utf8"/>
  </head>
  <body></body>
</html>

JavaScript

alert( document.documentElement.height );
alert( document.documentElement.clientHeight );