mozilla-release/testing/web-platform/tests/page-visibility/test_read_only.html

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8" />
        <title>Page Visibility API is Read Only</title>

        <script src="/resources/testharness.js"></script>
        <script src="/resources/testharnessreport.js"></script>
        <script type="text/javascript" src="resources/pagevistestharness.js"></script>

        <script type="text/javascript">
            setup({ explicit_done: true });
            function onload_test()
            {
                //check for feature definition first before attempting to overwrite
                pv_test();

                //Check document.hidden
                document.hidden = "new value";
                test_true(document.hidden !== "new value", 'document.hidden is read only.');

                //Check document.visibilityState
                document.visibilityState = "new value";
                test_true(document.visibilityState !== "new value",
                                "document.visibilityState is read only.");

                done();
            }
        </script>
    </head>
    <body onload="onload_test();">
        <h1>Description</h1>
        <p>This test validates that the Page Visibility attributes are read only.</p>

        <div id="log"></div>
    </body>
</html>