Screen-size based redirect

redirect.html

<!-- redirect based on screen width -->
<script type="text/javascript">
console.log('Screen width: '+screen.width);
if (screen.width <= 699 && !sessionStorage.getItem("redirecting")) {
window.location.replace("./m/index.html"); //path to your mobile site
}
</script>
<!-- redirect bypass on the mobile site
such that users don't get caught in a redirect loop
uses jQuery cuz mobile site is jQuery Mobile
but could easily be rewritten -->
<script>
$('a[href="http://example.com/"]').click(function(){sessionStorage.setItem("redirecting","1")});
//where example.com = your desktop site
</script>

Note: window.location.replace(url) is clearly more appropriate for JS redirects since it doesn't leave the redirecting page in the browser's history. See MDN's article on window.location.