rl-alternating-lr-tb writing mode (kind of boustrophedon style)

Hello !

Am I right that xsl 1.0 doesn't support rl-alternating-lr writing modes?
Such scripts did exist, I found references to ancient Hungarian runes and 
ancient Hawaiian.
It's not a real use case though, I'm just curious if lr-alternating-rl-tb is 
allowed as additional writing-mode value, why rl-alternating-lr-tb is not?

-- 
Oleg Tkachenko
eXperanto team
Multiconn International, Israel