Chain requirements are enforced, the level depends on conditions.

Official info is here: Caltrans Winter Driving Tips Winter Operations. Or you can call 1-800.427.7623 for current highway conditions.

From the website:
Chain Requirements
R1: Chains are required - snow tread tires allowed.
R2: Chains are required on all vehicles except four wheel drive vehicles with snow tires on all four wheels.
R3: Chains are required - all vehicles - no exceptions.
R1 and R2 are the most common conditions. The highway is usually closed before an R3 condition is imposed.

Along with the additional requirement that you carry chains.