Show a progress bar with every form that has a method of POST. Particularly nice if there's a file upload involved.

formProgress.js

// Licensed under a CC0 1.0 Universal (CC0 1.0) Public Domain Dedication: http://creativecommons.org/publicdomain/zero/1.0/
((win, doc)=>{ 'use strict';
    [...doc.querySelectorAll('form[method="post"]')].forEach((formElement)=>{
        function ajax (elem) {
            let progressBar = Object.assign(doc.createElement('progress'), {max:100, value:0});
            formElement.appendChild(progressBar);
            
            let url = elem.action, xhr = new XMLHttpRequest();
            xhr.open(elem.method, url, true);
            xhr.onload=(ev)=>{ win.location = url; };
            xhr.upload.onprogress=(ev)=>{ if (ev.lengthComputable) { progressBar.value = (ev.loaded / ev.total) * 100; } };
            xhr.send(new FormData(elem));
        }
        formElement.addEventListener('submit', (ev)=>{ ajax(this); ev.preventDefault(); }, false);
    });
})(this, this.document);