That said, I think it's great to see this support added to webkit. A diverse web is good for everyone.
So I suspect the causality is the other way around; there wasn't much demand for vertical form controls for the same reason you don't write emails in cursive.
Fortunately, for forms, I haven't seen any vertically written forms except school work.
This one. Think about it - they don't expect the reader to tilt his head 90 degrees to read it :)
Calligraphy, for example, conveys no extra information but is of higher stature.
That's enough reason to support it, imho. Japanese and Chinese are also routinely printed vertically, so that adds another billion and a half people who stand to benefit from it.
Most users of traditional Mongolian script are in China, where websites are in Chinese by default and Mongolian form elements are rare. They do exist though, and it would be nice if the search bar of this bookstore https://mn.dayangds.com/ could be vertical like everything else.