mozilla-release/testing/web-platform/tests/content-security-policy/form-action/form-action-self-allowed-target-blank.html

<!DOCTYPE html>
<html>

<head>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<meta http-equiv="Content-Security-Policy" content="form-action 'self'">
</head>

<body>
  <form action='/content-security-policy/support/postmessage-pass-to-opener.html'
        id='form_id'
        target="_blank">
  </form>

  <p>
    Test that "form-action 'self'" works correctly when the form uses
    target="_blank". If this test passes, a new window must open after pressing
    "submit".
  </p>
</body>

<script>
  async_test(t => {
    document.addEventListener('securitypolicyviolation', function(e) {
      t.unreached_func("Form submission was blocked.");
    });

    window.addEventListener('message', function(event) {
      t.done();
    })

    window.addEventListener("load", function() {
      document.getElementById("form_id").submit();
    });
  }, "The form submission should not be blocked by the iframe's CSP.");
</script>

</html>