See https://24ways.org/2016/css-writing-modes/ and http://labs.jensimmons.com/#writing-modes
Hyatt says that we currently have the infrastructure to do this quite easily, so the bulk of the work would be to hook up the new properties.
Hyatt says that these would probably best be implemented with two independent properties: one for line direction which could be rl, tb, and bt, and one for line orientation, which could be normal or flipped. Then writing-mode would be a shorthand for combining these two.
Hyatt says 1-2 days of work
'sideways-rl' and 'sideways-lr' are at-risk in the latest CR spec
and have been removed from the editor's draft
which lead me to say that supporting 'sideways-rl' and 'sideways-lr' is NOT required in order to claim compliance with CSS3 Writing Modes spec. In all fairness, P2 in this bug report does not seem justified in my opinion.