There are actually carve outs for these, although I agree the fact that the HUD is "transparent" makes it ambiguous (still, I think you can argue it "reduces" the driver's "clear view").

Without those carve outs, yes, they'd clearly be illegal.

Here's the relevant provisions:

> (b) This section does not apply to any of the following:

> (1) Rearview mirrors.

> (11) An electronic communication device affixed to the center uppermost portion of the interior of a windshield within an area that is not greater than five inches square, if the device provides either of the following:

> (A) The capability for enforcement facilities of the Department of the California Highway Patrol to communicate with a vehicle equipped with the device.

> (B) The capability for electronic toll and traffic management on public or private roads or facilities.

> (12) A portable Global Positioning System (GPS), which may be mounted in a seven-inch square in the lower corner of the windshield farthest removed from the driver or in a five-inch square in the lower corner of the windshield nearest to the driver and outside of an airbag deployment zone, if the system is used only for door-to-door navigation while the motor vehicle is being operated.

> (13) (A) A video event recorder with the capability of monitoring driver performance to improve driver safety, which may be mounted in a seven-inch square in the lower corner of the windshield farthest removed from the driver, in a five-inch square in the lower corner of the windshield nearest to the driver and outside of an airbag deployment zone, or in a five-inch square mounted to the center uppermost portion of the interior of the windshield. As used in this section, “video event recorder” means a video recorder that continuously records in a digital loop, recording audio, video, and G-force levels, but saves video only when triggered by an unusual motion or crash or when operated by the driver to monitor driver performance.