Usage no npm install needed!

<script type="module">
  import importHtmlEntry from 'https://cdn.skypack.dev/import-html-entry';
</script>

README

import-html-entry

Treats the index html as manifest and loads the assets(css,js), get the exports from entry script.

<!-- subApp/index.html -->

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>test</title>
</head>
<body>

<!-- mark the entry script with entry attribute -->
<script src="https://unpkg.com/mobx@5.0.3/lib/mobx.umd.js" entry></script>
<script src="https://unpkg.com/react@16.4.2/umd/react.production.min.js"></script>
</body>
</html>

import importHTML from 'import-html-entry';

importHTML('./subApp/index.html')
    .then(res => {
        console.log(res.template);

        res.execScripts().then(exports => {
            const mobx = exports;
            const { observable } = mobx;
            observable({
                name: 'kuitos'
            })	
        })
});