Tinting is illegal per se but the law allows glazing.

Federal standards:

Federal Motor Vehicle Safety Standard No. 205 (49 C.F.R. 571.205), including the specified minimum light transmittance of 70 percent and the abrasion resistance of AS-14 glazing, as specified in that federal standard.

State standards:

In CA, the law says:

Devices utilizing transparent material shall be green, gray, or a neutral smoke in color and shall have a luminous transmittance of not less than 35 percent.

Sun screening devices meeting the requirements of Section 26708.2 installed on the side windows on either side of the vehicle'sfront seat, if the driver or a passenger in the front seat has inhis or her possession a letter or other document signed by a licensed physician and surgeon certifying that the person must be shaded fromthe sun due to a medical condition, or has in his or her possession, a letter or other document signed by a licensed optometrist certifying that the person must be shaded from the sun due to avisual condition. The devices authorized by this paragraph shall not be used during darkness.