Cal. Government Code § 50026

The legislative body of any local agency, chartered or general law, which is otherwise authorized by law or charter to impose any tax on the privilege of earning a livelihood by an employee or any other tax, fee or charge on or measured by the earnings, or any part thereof, of any employee, shall not impose any such tax, fee or charge on the earnings of any employee, when such employee is not a resident of the taxing jurisdiction, unless exactly the same tax, fee or charge at the same rate, with the same credits and deductions, is imposed on the earnings of all residents of the taxing jurisdiction who are employed therein. This section shall not be construed as authorizing any tax prohibited by Section 17041.5 of the Revenue and Taxation Code or any other provision of law, nor shall it be construed so as to prohibit the levy or collection of any otherwise authorized tax upon a business measured by or according to gross receipts.