[simonbrown]

If it loads the page in an iframe, wouldn't future loads give a false positive?

Also, does this work for Google+ (I don't have an account)? I'd be surprised if it did, since it uses X-Frame-Options.

[futmacl]

If you allow the load to complete, yes. But the idea here is to very quickly decide that you're not getting a cached copy, and abort the request (by changing src=) before the browser has a chance to read anything back and figure out what to do with it.