I must have done something wrong. I filled everything out and it had me print a form that I'm supposed to mail to my registrar in order to register... which of course is registering via mail and not online.
I'm not sure what conditions led to that outcome. My successful registration (using my license #) yielded the following message: Congratulations! Your DMV signature has been retrieved and attached to your voter registration form and will be provided to your county elections office. Your form is now complete. Thank you for registering to vote! Please print a receipt for your records.
Yah that's the problem... I don't have a CA license. I'm a student but I'm from a different state, whose voter registration says to register to vote in CA. Oh well... mail works.
They (the DMV) must not have your signature on file. Unfortunately they need this as part of "the rules"...which aim combat voter fraud. It's inconvenient, but at least they fill out the form and do their best.