Add some additional css and html on runtime. (I am using this method for landing pages)

inject-style-and-html.js

(function () {
    const base_path = 'https://example.com/base/path/';
    function addStyle(name) {
        const path = base_path + 'css/' + name + '?v=' + Date.now();
        const link = document.createElement('link');
        link.rel = 'stylesheet';
        link.type = 'text/css';
        link.href = path;
        document.head.appendChild(link);
    }

    addStyle('stylesheet.css');

    function addHtml(filename, target_selector) {
        const path = base_path + 'html/' + filename + '?v=' + Date.now();
        const target = document.querySelector(target_selector);
        console.log('target (' + target_selector + ')', target);
        if (!target) {
            return;
        }
        const xhr = new XMLHttpRequest();
        xhr.open('GET', path);
        xhr.onreadystatechange = function () {
            if (xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {
                target.innerHTML = xhr.responseText;
            }
        };
        xhr.send();
    }

    addHtml('source.html', '.target-element-on-page');
}());