example.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>Example</title>
  <style>
    html,
    body,
    #root {
      margin: 0;
      width: 100%;
      height: 100%;
    }
  </style>
</head>

<body>
  <div id="root"></div>
  <script src="./example.js"></script>
</body>

</html>