gistfile1.txt

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

(function() {
    'use strict';

function matches() {
	const h1 = document.querySelector("h1");
	return (
		h1 &&
		h1.textContent === "404 Not Found");
}

if (matches()) {
	const mdnLink = document.createElement("a");
	mdnLink.href = `https://developer.mozilla.org/${location.pathname}`;
	mdnLink.textContent = "View on MDN";
	document.body.appendChild(mdnLink);
}
})();