Here's a link to the law for California
"Material Obstructing or Reducing Driver's View"
There are exceptions which allow the top 4-inches of the windshield and front side windows to have aftermarket tint applied.
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(a), clear, colorless, and transparent material may be installed, affixed, or applied to the front side windows, located to the immediate left and right of the front seat if the following conditions are met:
(1) The material has a minimum visible light transmittance of 88 percent.
(2) The window glazing with the material applied meets all requirements of 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.
(3) The material is designed and manufactured to enhance the ability of the existing window glass to block the sun's harmful ultraviolet A rays