Is it worth supporting multiple direction variants? For instance, switching a CJK wiki's display between horizontal-LTR and vertical-top-to-bottom-RTL?
Life gets much simpler if we don't allow switching, and only use vertical for languages that require it.
I could see it being useful to have the option available for those who might be slightly more comfortable reading vertically, but I think that right-to-left vertical script could be a much lower priority than getting left-to-right vertical text working.
(According to Omniglot, "In Taiwan [Chinese] is often written vertically, while in China and Singapore it is usually written horizontally.", so it might make sense to have the Taiwanese variant on the Mandarin Chinese Wikipedia default to vertical if that's possible at some point.)
I think Taiwanese are comfortable with reading horizontally online. But some book and all of ancient Chinese books are written horizontally. A vertical mode should be good for all Chinese use case.