Most Countries do not have a specific law that states in such and such Act the body must be made only of certain metals.
They have sets of laws (which is part of the problem)1/ Basic standards Headlights max power and alignment brakelights must be Red at least 2 of them and usually a wattage None of which dictates for on lights what the lumens must be (France and yellow headlights come to mind)
For this country steering wheel must be left or right etc
General comments like to meet standards of certifying agency at time of manufacture (rarely retroactive)
Minimum of from this date all cars must have a seat belt Changes in requirements over time dealt with by certifying agency (because it is quicker usually and done with consultation with manufacturers they regularly deal with)
Often these laws are varied or cover specific aspects, lights seat belts, emmisions SEPARATELY
Then there are different classifications for types of vehicles2/ General Laws from treaties and international agreement
For example layout of which side wipers and lights should be on steering column was at one time haphazd but change by agreement during 70's era.
Acceptance of certain countries/regional authority vehicle certification standards as complete or starting point.3/ General Guidelines for Certifying agemcy its goals and limits With the power to set standards usually in forms of publications
Often from a general law about a Vehicle Category(s) and the minimum MANUFACTURING requirements and which certifying agencies are involved.
Then you have usually Insurance Body or Motor Manufacturer Body lead initiatives like
Thatcham Group and Euro NCAP (now be copied around the world China, India.....)
Which partly testing before production, during sales AND analysis of crashes over time by model and type of incident.
These also go into Driver Assitance Systems (Lane Support, Speed Assist, Collision Mitigation...) no doubt before long driverless systems.
MISRA which most people here have heard of
Most of these have for decades been more about mechanical, electrical and fire safety, simply due to the fact that that was what the major components were.
Rarely do they consider drivers need retraining.