<script>
    try {
        const initialHistoryLength = window.history.length;

        history.pushState({}, "hello", "history-pushstate-iframe.html#hello");

        parent.postMessage(
            "history object length has changed by " +
                (window.history.length - initialHistoryLength),
            "*"
        );
    } catch (e) {
        parent.postMessage("ERROR:" + e, "*");
    }
</script>