chromium/third_party/blink/web_tests/fast/frames/srcdoc/srcdoc-beats-src.html

<script>
if (window.testRunner)
    testRunner.dumpAsText();
</script>
<iframe src="data:text/html,<script>alert(/FAIL/);</script>" srcdoc="PASS"></iframe>
This test passes if it doesn't alert FAIL.