Firefox #1475000

index.js

"use strict";

const http = require("http");

const server = http.createServer((req, res) => {
	if (req.url.includes("slow.js")) {
		setTimeout(() => {
			res.setHeader("Content-Type", "text/javascript");
			res.end("void 0;");
		}, 5000);
	} else {
		res.statusCode = 404;
		res.setHeader("Content-Type", "text/plain");
		res.end("Not found");
	}
});

server.listen(8888);

test.html

<p>This should get erased.</p>
<button onclick="document.open();">document.open()</button>
<script src="http://localhost:8888/slow.js"></script>
<script>
  alert("You were too slow! Hit stop before this message.");
</script>