file_frame_src_child_governs.html

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<html>

<head>

  <meta charset="utf-8">

  <meta http-equiv="Content-Security-Policy" content="child-src https://example.com">";

</head>

<body>

<iframe id="testframe"></iframe>

<script type="text/javascript" src="file_frame_src.js"></script>

</body>

</html>