Not sure, that's what I'm wondering. Sounds like a good solution if they automatically do fallback. I also wonder how browsers behave depending on how the server at a particular IP address is responding, as they can respond in different ways. (i.e. a server might respond with an error, accept a request but timeout on a response, or appear totally unreachable)

Any HA solution I've seen that attempts to reliably achieve this five nines capability relies on network-level things like virtual IP's and what not. And I don't consider it a five nines solution if only some customers can access it. How the browser behaves in this case could be critical depending on how your visitors use the site. I would not consider a site "up" if it's only available to some people and not others.