It is non-standard, and has some maintenance cost.
Pull request: https://github.com/WebKit/WebKit/pull/3917
<rdar://problem/99698393>
Searching 'webkit-horizontal-bt' on GitHub does not seem to show much and mostly in 'WebKit' repo. Firefox never implemented this and Blink removed last code in 2017 (if I go by this commit in 2017): https://chromium.googlesource.com/chromium/src.git/+/3621a3132bef560f740f532aa24a76230f0a7ac1 If we have any internal Apple metrics for use case, I think we should give it another try. @Tim - any metric or use cases within Apple Apps?