mdn-link.js

// ==UserScript==
// @name         Add MDN link
// @namespace    http://tampermonkey.net/
// @version      2025-01-08
// @description  Add MDN links to PR preview 404 pages
// @author       wbamberg
// @match        https://*.content.dev.mdn.mozit.cloud/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// ==/UserScript==

(function () {
  "use strict";

  function run() {
    const h1 = document.querySelector("h1");
    if (
      h1 &&
      ( h1.textContent === "404 Not Found" ||
        h1.textContent === "Page not found" )
     ) {
        const mdnLink = document.createElement("a");
        mdnLink.href = `https://developer.mozilla.org${location.pathname}`;
        mdnLink.textContent = "View on MDN";
        h1.insertAdjacentElement("afterend", mdnLink);
      }
    }

  window.setTimeout(run, 1000);
})();