Tampermonkey Userscript to use HTML Gmail as the default

userscript.js

// ==UserScript==
// @name         Use HTML Gmail as Default
// @namespace    http://tampermonkey.net/
// @version      0.2
// @description  Set the HTML version of Gmail as the default, and add an account switcher
// @author       Michael M
// @match        https://mail.google.com/mail/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// @run-at document-start
// ==/UserScript==

(function() {
    'use strict';

    // rewrite the URL to use the HTML version
    if (!location.href.includes('/h')) {
        location.href = location.href + '/h/';
    }

    window.onload = function() {
        // get the current user ID from the URL
        const currUser = location.href.match(/\/u\/(\d)/)[1];

        // add a switcher to quickly switch accounts
        const switcher = document.createElement('a');
        switcher.style.margin = '10px';
        switcher.textContent = 'Switch Account';
        switcher.href = location.href.replace(/\/u\/\d/, '/u/' + currUser + 1);
        document.getElementsByClassName('gbi')[0].appendChild(switcher);
    };
})();