https://medium.com/@eartz_hc/dynamic-import-of-es6-modules-with-fallback-to-systemjs-c72b30b8225e

dynamic-import.html

<!-- see it in action here : https://github.com/rollup/rollup-starter-code-splitting/blob/master/public/index.html -->
<!-- Browsers with dynamic import support -->
<script type="module">
    window.esDynamicImport = true;
    // this will throw if dynamic import is not supported
    import("/js/es/entrypoint.js").then(function(m) {
        // do something
    });
</script>

<!-- browsers with ES6 modules but no dynamic import support -->
<script type="module">
// if the module script tag throws, window.esDynamicImport will be undefined
if (!window.esDynamicImport) {
    // get the nomodule scripts and load (or inline) them
    const noms = Array.prototype.slice.call(document.getElementsByTagName('script'), 0).filter((x) => x.hasAttribute('nomodule'));
    function nextLoad (nom, s) {
        if (!(nom = noms.shift())) return;
        s = document.createElement('script');
        if (nom.src)
            s.src = nom.src, s.addEventListener('load', nextLoad), s.addEventListener('error', nextLoad);
        else
            s.innerHTML = nom.innerHTML;
        document.head.appendChild(s);
        if (!nom.src) nextLoad();
    }
    nextLoad();
}
</script>

<!-- No ES6 modules support : fallback to systemJS -->
<script nomodule src='https://unpkg.com/[email protected]/dist/system-production.js'></script>
<script nomodule>
    System.import("/js/system/entrypoint.js").then(function(m) {
        // do something
    });
</script>