srcdoc-child-frame.html

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<head>

  <meta http-equiv="Content-Security-Policy" content="img-src 'none'">

</head>

<body>

  <script>

    var i = document.createElement('iframe');

    i.srcdoc=`<script>

                window.addEventListener('securitypolicyviolation', function(e) {

                  if (e.violatedDirective == 'img-src') {

                    top.postMessage('img blocked', '*');

})

              </scr` + `ipt>

              <img src='/content-security-policy/support/fail.png'

                onload='top.postMessage("img loaded", "*")'/>`;

    i.id = "srcdoc-frame";

    document.body.appendChild(i);

  </script>

</body>