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chromium/third_party/blink/web_tests/external/wpt/html/semantics/embedded-content/the-iframe-element/support/iframe_sandbox_029.htm 

<!DOCTYPE html>
<html>
<head><title>Page with access to document.cookie</title>
</head>
<body>
    <div>Cookie Read: <span id="readCookie"></span></div>
    <script type="text/javascript">
        try
        {
            cookie = document.cookie;
            document.cookie = "name=browser";
            parent.window.postMessage("cookies are R/W", "*");
        }catch(e)
        {
            parent.window.postMessage("cookies are not R/W", "*");
        }
    </script>
</body>
</html>