website/js/includeHTML.js

let z = document.getElementsByTagName("*");

for (let i = 0; i < z.length; i++) {
    let element = z[i];
    let externalFileURL = element.getAttribute("includeHTML");
    if (externalFileURL) {
        let xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function() {
            if (this.readyState == 4) {
                if (this.status == 200) { element.innerHTML = this.responseText; }
                if (this.status == 404) { element.innerHTML = "Page not found."; }
                element.removeAttribute("includeHTML");
            }
        }
        xhr.open("GET", externalFileURL, true);
        xhr.send();
    }
}