JavaScript Universal Bundle Loader, as explained in this Medium post.
<!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>JS Universal Bundle Loader</title>
<!-- targeting native async/await as ES2017 -->
<script type="module">window.ESM=!0;import'./es2017.js';</script>
<!-- targeting ES2015 with ES5 fallback -->
<script nomodule defer src="ubl.js"></script>
</head>
<body></body>
</html>