Microsoft's Ajax CDN tumbles worldwide (opens in new tab)

CDN, with local fallback:

    <script src="//ajax.aspnetcdn.com/ajax/jQuery/jquery-1.x.x.min.js" type="text/javascript"></script>
    <script type="text/javascript">
    window.jQuery || document.write(unescape('%3Cscript src="/scripts/jquery-1.x.x.min.js"%3E%3C/script%3E'))
    </script>

I think the idea is that hosts like Google (and ostensibly, Mircosoft) are going to have availability that far exceeds your run-of-the-mill website. Those big hosts generally are going to provide five 9's (or more) availability, whereas %your_random_site% is probably going to be on the order of three to four 9's, at best. So more often than not, the point of failure is going to be your website...not the CDN.

If you have the infrastructure ($$$) to support your own web services on the order of 99.999%+, then by all means host it yourself, otherwise you're most likely never going to have a problem with it (especially if you include a fallback method).

This was what I was going to suggest also... but...

Whatever happened to using progressive enhancement techniques which meant the site would continue to work, albeit with basic functionality, for users even without JavaScript?

I understand for some sites this is not possible, but for a lot of sites it would be perfectly acceptable...

This is exactly my thought on the matter. It's cool that larger sites will host these scripts for you, but why should I make my site dependent on another just for a negligible improvement in speed / caching efficiency?

My feeling is that if you're going to make your site's uptime dependent on another, at least make it for something worthwhile that you can't host yourself, either because it's too demanding or too proprietary.

"More generally, I have never understood why people use these third party CDNs for important sites."

Perhaps its not a core-competency of the company? When you're a small company you only get a few employees to work with maybe none, so you find ways to get the stuff you don't know how to do, done. Whether its contract work for design, or outsourcing operations to the cloud.

How often does your average user browse via an anonymous proxy? I doubt most would even know what the hell you are talking about. I can understand for your more clued up or power user, but you give the average user too much credit.