The document.write method is one you generally want to avoid, but it works best in this case:
<!-- Use Google CDN for MooTools, local Moo as a callback --> <script src="//ajax.googleapis.com/ajax/libs/mootools/1.4.1/mootools-yui-compressed.js"></script> <script>!window.MooTools && document.write(unescape('%3Cscript src="/js/mootools-yui-compressed.js"%3E%3C/script%3E'))</script>
In the event that the CDN fails, your local file is instead used. Simple but clever fallback, yes? The likelihood that Google or other CDNs is down is small, but since you don't control the other server, there's always that risk! Play it safe!