blog/theme/navbar.js

document.addEventListener('DOMContentLoaded', function () {
    fetch('/includes/navbar.html')
        .then(function (res) { return res.text(); })
        .then(function (html) {
            var wrapper = document.createElement('div');
            wrapper.innerHTML = html.trim();
            document.body.insertBefore(wrapper.firstChild, document.body.firstChild);
        });
});