gistfile1.txt

function matches() {
	const h1 = document.querySelector("h1");
	return (
		h1 &&
		h1.textContent === "404 Not Found" &&
		location.host.endsWith(".content.dev.mdn.mozit.cloud")
	);
}

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