Online DoS shouldn't really be a problem, since you should be rate limiting logins on a per-IP basis to prevent online brute-force anyway. Of course, typically nobody does that.

'Server relief' is dumb... correctly implementing a 'hashcash'-like scheme to prevent excessive CPU resource commitment is non-trivial. It will require either persistent pre-login server state, and/or cryptographic IP/session binding, both of which open up a second DoS vector.

Implementing Scrypt etc in the browser doesn't completely prevent DoS, and sacrifices your ability as an operator to ensure passwords are being appropriately salted and munged (because of third party clients, unauthorised API users etc). Plus, attackers can just start using malicious webpages or botnets to crowdsource their hashing efforts which, because it scales horizontally, could actually speed things up for an attacker if you're server isnt doing any rate-limiting on the final hash+authentication stage.