It's unclear to me how your random salt would work. From my understanding, you're suggesting smth like:

register: send (username, user_salt, HMAC(user_salt, pwd))

login: send (username). retrieve user_salt. retrieve a server_salt generated randomly. send HMAC(server_salt, HMAC(user_salt, pwd))

But now your password is effectively just HMAC(user_salt, pwd), and the server has to store it in plaintext to be able to verify. Since plaintext passwords in the db are bad, this solution doesn't sound too attractive, unless you were suggesting something else.