JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <h1 id="innerWidth"></h1>
</body>
</html>

CSS

@media (min-width: 768px) {
  body {
    background: red;
  }
}

@media (max-width: 767px) {
  body {
    background: green;
  }
}

JavaScript

function update() {
  document.getElementById("innerWidth").innerText = String(window.innerWidth);
}

update();
window.onresize = update;