View source for WAA suggesting user name

==A brief description of what is proposed==

An addition to the webauth protocol to allow the waa to suggest (or mandate) a user name for the user to use.

==An explanation of how it would be useful==

There are circumstances (my Raven/OpenID bridge being the only one that comes to mind) where a WAA knows that there is only one user name that will be a satisfatory response to an authentication request.  In these circumstances, the user would be helped if Raven presented them with a login box with the user name already filled in.  It might even be reasonably for it to prevent the user changing the user name, but I think that would probably cause too much confusion.

==One or more suggestions of how it could be implemented==

An additional parameter added to the authentication request:

   principal [OPTIONAL] If present, indicates a suggested identity
             to offer the user.  The WLS MAY use this to seed a login
             form.

This would also require a change to the <tt>ver</tt> parameter.

Raven would seed the "User-id' box with the supplied identity, in the same way as it usually does from a cookie, arranging to put the input focus in the password box if possible.  Otherwise, its behaviour would be unchanged.