<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<script src="libs/bower_components/jquery/dist/jquery.min.js"></script>
<script src="libs/bower_components/underscore/underscore.js"></script>
<script src="libs/bower_components/backbone/backbone.js"></script>
<script src="libs/bower_components/backbone.radio/build/backbone.radio.js"></script>
<script src="libs/bower_components/backbone.eventrouter/dist/backbone.eventrouter.min.js"></script>
<script src="libs/bower_components//backbone.marionette/lib/backbone.marionette.min.js"></script>
<!--
<script src="libs/node_modules/path-to-regexp/index.js"></script>
<script src="libs/node_modules/cherrytreex/cherrytree.js"></script>
<script src="libs/node_modules/marionette.routing/marionette.routing.js"></script>
<script type="module" src="js/test.mjs"></script>-->
<script type="module" src="js/start.js"></script>
<link rel="stylesheet" href="css/styles.css">
<title></title>
</head>
<body>
<div id="mwr">
</div>
</body>
</html>
import Bb from 'backbone';
import { Application } from 'backbone.marionette';
'use strict';
exports.Application = Application;
<script type="module" ...
не нужно указывать ".js"