 class="sentinel" style="right:100vw; top:-200vh; height:500vh;">Sentinel Left</div>
        <div class="sentinel" style="bottom:100vh; left:-200vw; width:500vw;">Sentinel Top</div>
        <div class="sentinel" style="left:200vw; top:-200vh; height:500vh;">Sentinel Right</div>
        <div class="sentinel" style="top:200vh; left:-200vw; width:500vw;">Sentinel Bottom</div>
    </div>
    <a href="?flag">Turn flag on/off</a>
</body>

</html>