At least in my state, there is a law that restricts the location of headlights to between 22 and 54 inches from the ground. 54 inches is quite tall, though, I think that a lot of cars have roofs that are shorter than 4.5 feet. I'd love to see a much lower upper limit.
I don't think there's a limit to how bright they can be. The law limits the lights to "70 watts", which I believe is intended to limit brightness but misses the mark. I bet the law was passed back when headlights were incandescent.
LEDs are around 10x more efficient than incandescent bulbs. A 100W incandescent equivalent LED bulb typically consumes 10-12W. A 70W LED would put out as much light as 700W of incandescent.
Traditional automotive headlamps are halogens and much more efficient than conventional incandescents. It was only in the last decade that LEDs beat halogens on lumens per watt.
I don't think there's a limit to how bright they can be. The law limits the lights to "70 watts", which I believe is intended to limit brightness but misses the mark. I bet the law was passed back when headlights were incandescent.