BrowserID provides a way to say to a site "I own this email address", as well as a protocol for the site to verify such an assertion. What the site chooses to

BrowserID provides a way to say to a site "I own this email address", as well as a protocol for the site to verify such an assertion. What the site chooses to do with the information is up to them. It can be used for account creation or signup, but works equally well for authentication to an existing account. In the mockmyid case, you are saying "I own the address xyz@mockmyid.com". But the MockMyID server will happily let anyone make such an assertion, so you get a simple kind of mock identity. Of course, you shouldn't use that as your identity on any sites that you care about. Mail to anything@mockmyid.com doesn't go anywhere, and there's nothing to prevent other people from using the same @mockmyid.com address. But it's a neat example of the sort of thing that is possible.