I decided to check the Calif. Vehicle code and my error:
28071. Every passenger vehicle registered in this state shall be equipped with a front bumper and with a rear bumper. As used in this section, "bumper" means any device designed and intended by a manufacturer to prevent the front or rear of the body of the vehicle from coming into contact with any other motor vehicle. This section shall not apply to any passenger vehicle that is required to be equipped with an energy absorption system pursuant to either state or federal law, or to any passenger vehicle which was not equipped with a front or rear bumper, or both, at the time that it was first sold and registered under the laws of this or any other state or foreign jurisdiction.
I'm not sure but if you bought your car new and with no bumpers and could prove that fact or, if you registered your car in Calif. and bought it out of state and it had no bumpers and could prove that fact, I suppose you may still get a citation but you may be successful if you go to trial and can show proof of the last sentence in section 28071 as seen above.
Secondly, although a lot of you argue about how bad it may look, or you've driven cars for 40 years without a front license plates etc etc etc, the question was,,, is it legal in California.....NO !!!
It's like speeding, illegal turns, rolling through a stop sign etc etc etc. Sometimes you get caught, often times you don't.
And when you do get caught, sometimes you just get a warning.
Sometimes things that are illegal are overlooked like tinting, no bumpers, no front license plate, burned out light bulbs etc etc.
Sometimes no matter how nice you are to the Law Enforcement Officer, you'll get a citation.
Sorry about the misinformation on the bumpers but I've been retired for over 7 years and a lot of the Vehicle Code sections have dribbled out of my brain through my ears...