Core Site Redirect using feature detection

gistfile1.txt

    <script>

        var ef = "?_escaped_fragment_=";

        if (!('querySelector' in document)
             || !('localStorage' in window)
             || !('addEventListener' in window)
            || !('matchMedia' in window)) {

            if (window.location.href.indexOf("#!") > 0) {
                window.location.href = window.location.href.replace("#!", ef);
            } else {
                if (window.location.href.indexOf(ef) < 0) {
                    window.location.href = window.location.href + ef;
                }
            }

        } else {

            if (window.location.href.indexOf(ef) >= 0) {
                window.location.href = window.location.href.replace(ef, "#!");
            }
        }

    </script>