Well, there is always the html5 <keygen> tag?

http://www.w3.org/TR/html-markup/keygen.html

https://developer.mozilla.org/en-US/docs/HTML/Element/keygen

What!? No demoscene music in my <keygen> ?

SSL includes a feature called client-side certificates: http://pilif.github.io/2008/05/why-is-nobody-using-ssl-clien...

Actually, there's a W3C WG that works (worked?) on a protocol called WebID, that uses public keys to authenticate users, and servers are not identity providers but just serving profile information.

http://www.w3.org/2005/Incubator/webid/spec/#authentication-...