This article is from the California Driving (and Surviving) FAQ, by "George J Wu" firstname.lastname@example.org with numerous contributions by others.
from email@example.com (John Hunley) on 14 Dec and 15 Dec 1992:
The applicable paragraph in the CVC is 26708. It's too long to quote here
in full, but basically what it says (disclaimer: this is my own personal
interpretation, I'm not a lawyer, don't come running to me if you get
nailed) is that you may not operate with "any object or material placed,
displayed, installed, affixed, or applied upon the windshield or side or
rear windows." Side windows to the rear of the driver are exempted
(26708b4), as is the rear window IF you have mirrors on both left- and
right-hand sides (26708b8). Tinted safety glass is permitted by 26708.5b.
Therefore, the basic distinction is whether you have tinted glass or tinting
that is stuck onto the glass. There's no mention of "factory" vs. "third
party." A third-party tint job would be legal if it was done by replacing
the windshield and front windows with tinted safety glass, rather than by
sticking or painting something onto the existing glass.
An interesting side note is that 26708a3 specifically includes snow and ice
as an obstruction covered under 26708. So you can get a ticket for 26708
for having snow or ice on your windshield or front windows, as well as
stick-on tinting. Same violation.
The pull-down blinds are permitted ONLY if the driver or front passenger has
a signed document from a physician or optometrist (CVC 26708b10) stating
that they are required due to a medical or visual condition. CVC 26708b3
allows "signs, stickers, or other materials which are displayed in a 7-inch
square in the lower corner of the windshield ... [or] ... rear window
farthest removed from the driver, or ... in a 5-inch square in the lower
corner of the windshield nearest the driver."
Also exempted are such things as rearview mirrors, rear window wiper
hardware, rear trunk lid handles and hinges, destination signs on busses,
magnifying or wide-angle lenses on the passenger side window of a truck, and
of course the standard equipment sun visors.