diyhrt/diyhrt.wiki/nav.js

fetch('nav.html')
    .then(res => res.text())
    .then(text => {
        let oldElem = document.querySelector("script#replace_with_navbar");
        let newElem = document.createElement("div");
        newElem.innerHTML = text;
        oldElem.parentNode.replaceChild(newElem, oldElem);
    })