chromium/third_party/blink/web_tests/http/tests/security/mixedContent/resources/frame-with-insecure-eventsource.html

<script>
var es = new EventSource("http://example.test:8000/security/mixedContent/resources/simple-event-stream.pl");
es.addEventListener("error", function() {
    if (window.opener)
        window.opener.postMessage('done', '*');
});
</script>