I know this thread focuses on California, but for others, keep in mind that each state is different on the requirement for a front plate.
Here in Washington State, two plates are a must on "normal" cars and trucks...and it's not just a fixit ticket if caught. It's a $250 fine. And it is enforced (ask me how I know
).
If your car is old enough to run Year of Manufacture Plates, only the rear is required. The requirements and restrictions for YOM plates are the same as for "Collector Car" plates, however.
Check your individual state laws before assuming anything.