We have several shared machines which we smei-regularly login to Phabricator from. Since the machines are shared we regularly log-in/out and I've noticed that the login page doesn't auto-focus the username field. On setups with multiple authenticators I don't think this would have well-defined behavior, in which case maybe just auto-focusing the first one allowing for tabbing to subsequent forms.
Observed on Firefox 39, OSX 10.10