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chromium/third_party/blink/web_tests/fast/loader/resources/page-dismissal-modal-dialogs-iframe.html 

<!DOCTYPE html>
<html><head><meta charset=utf-8><title>Page Dismissal Modal Dialogs iFrame</title><script>
parent.registerEvents("iFrame", window, parent);
</script></head><body><div>
<span onclick="parent.showMessages('iFrame click')" id=anchor>anchor</span>
</div></body></html>