These are just 2 threads taken from a simple search of the word “HID” there are many more.
“Part 564 - Replaceable Light Source Information (Effective 1-1993) Appendix A to Part 564 - Information To Be Submitted for Replaceable Light Sources
This part requires the submission of dimensional, electrical specifications, and marking/designation information, as specified in Appendix A and Appendix B of this part, for original equipment replaceable light sources used in motor vehicle headlighting systems.
The purposes of this part are achieved through its Appendices:
(a) The purposes of Appendix A of this part are to ensure:
(1) The availability to replacement light source manufacturers of the manufacturing specifications of original equipment light sources so that replacement light sources are interchangeable with original equipment light sources and provide equivalent performance, and
(2) That redesigned or newly developed light sources are designated as distinct, different, and noninterchangeable with previously existing light sources.
(b) The purposes of Appendix B of this part are to ensure:
(1) That original equipment light sources are replaceable and that replacement light sources provide equivalent performance, and
(2) That redesignated or newly developed light sources are designated as distinct, different, and noninterchangeable with previously existing light sources.
This part applies to replaceable light sources used as original equipment in motor vehicle headlighting systems.”
Original hid bulbs were designed in D1, D2, D3 bulb sizes. To comply with the law that they can not fit in am existing Bulb socket. You will not find any reputable company making a h4, 9005, 9004, ect hid bulb as it is not legal. It is also illegal to import hid kits into the US for this reason