chromium/third_party/blink/web_tests/http/tests/security/mixedContent/resources/frame-with-redirect-https-to-http-frame.html

<script>
function frameLoaded() {
  if (window.opener)
    window.opener.postMessage('done', '*');
}
</script>
<iframe onload="frameLoaded()"
        src="https://127.0.0.1:8443/security/resources/redir.php?url=http://example.test:8080/security/mixedContent/resources/boring.html"></iframe>