One final (and extremely promising) option for authentication is Mozilla Persona. While it currently offers simple email-based authentication in a cross-browser way, the long-term goal of the project is to bring authentication directly into the browser itself. If this goal is realized, it may become possible to have a trusted authentication scheme that doesn't rely on any server whatsoever.
Persona received less adoption than we were hoping for by this point. A summary of the lessons learned is available in Persona’s “After Action Review” (AAR).
Sadly, you're right, I should probably revise the article. User agent auth is still a dream of mine, but it looks like it's not going to come to pass just yet.
And since March, 7th:
So I'm not sure how optimistic the aforementioned article can still be about the future of Persona. Thoughts?