sideways-left is for Latin and other non-East Asian vertical flow, while other values (mixed, upright, sideways-right) are important values for East Asian vertical flow.
sideways has dependency on sideways-left, and thus is implemented differently from the spec by 3 implementations (AH, Blink, WebKit)
Mongolians must use sideways-right, not sideways, but the 3 implementations allow both.
Proposals
sideways-left
Keep as is
Mark at risk and put a note for attention not to implement the sideways value if you don't implement this value
Defer to Level 4
sideways
Keep as is
Mark at risk and put a note for attention not to implement this value differently from the spec
Change the spec to follow the interoperable 3 implementations (same behavior as sideways-right)