I haven't look at the specs deeply, but it would be nice to have a system that did not need any kind of server at all, but the browser itself could be the Persona identity provider. The actual local data needed to pull it off could be replicated (encrypted) to cloud storage so it would work across all your devices and browsers, but the actual profile data itself would never be readable by the servers. I started looki…
That would end up requiring your identity to be a key of some sort, though, since ultimately the thing you're logging into needs to check _something_. Email-like things are useful because somebody has to control the domain; if you don't have that, it's harder to establish something. Of course, you can say things like "Hi, I'm random prime number 534473", but that's much less memorable...
The point is, nothing prevents the site from knowing me as 70BD6432E50DFB65FF679B32364C3C7840DE4453, but displaying me everywhere as "Aleksey".