[css-writing-modes] 'direction' vs. vertical text

In "Introduction to Writing Modes", the inline base direction is defined 
such that it exists in all writing modes. However, the current 
definition of the 'direction' property seems to assume an horizontal 
writing mode:

> Values for this property have the following meanings:
>
> ltr
>     Left-to-right directionality.
> rtl
>     Right-to-left directionality.

What do these values mean for vertical text?

Should they map to top-down and bottom-up respectively, or should the 
inline-start direction always be up, or something else?

(This may be defined in another part of the spec that I haven’t read 
yet, but it should at least be linked from here, if not defined here.)

-- 
Simon Sapin