maximum-downlink.js

// register the service worker
navigator.serviceWorker.register('/worker.js').then(
    function(reg) { console.log('Installed successfully', reg) },
    function(err) { console.log('Worker installation failed', err) }
);

// ... worker.js
self.addEventListener('fetch', function(event) {
    var requestURL = new URL(event.request);

    // Intercept same origin /video/* requests
    if (requestURL.origin == location.origin) {
        if (/^\/video\//.test(requestURL.pathname)) {
            // append the MD header, set value to NetInfo's downlinkMax:
            // http://w3c.github.io/netinfo/#downlinkmax-attribute
            event.respondWith(
                fetch(event.request.url, {
                    headers: { 'MD': navigator.connection.downlinkMax }
                })
            );
            return;
        }
    }
});