[css-writing-modes] RTL scripts in vertical text

The Unicode Bidi spec [1] describes two ways of handling RTL scripts in
vertical writing modes.

(a) Apply only the first part of the bidi algorithm, which resolves bidi
levels. Use these levels not to reorder the line, but instead to determine
whether to rotate sideways left or sideways right, so that the line is
uniformly ordered from top to bottom:

In the case of vertical line orientation, the Bidirectional Algorithm is
> still used to determine the levels of the text. However, these levels are
> not used to reorder the text, because the characters are usually ordered
> uniformly from top to bottom. Instead, the levels are used to determine the
> rotation of the text. Sometimes vertical lines follow a vertical baseline
> in which each character is oriented as normal (with no rotation), with
> characters ordered from top to bottom whether they are Hebrew, numbers, or
> Latin. When setting text using the Arabic script in vertical lines, it is
> more common to employ a horizontal baseline that is rotated by 90°
> counterclockwise so that the characters are ordered from top to bottom.
> Latin text and numbers may be rotated 90° clockwise so that the characters
> are also ordered from top to bottom.


(b) Use the bidi levels to reorder the line, just as in horizontal writing
modes:

The Bidirectional Algorithm is used when some characters are ordered from
> bottom to top. For example, this happens with a mixture of Arabic and Latin
> glyphs when all the glyphs are rotated uniformly 90° clockwise.


I see how to do (b) with the current spec, but I don't see how to do (a)
(without explicitly marking up each bidi run). I was wondering whether this
capability was omitted intentionally, and, if so, why.

James

[1] http://www.unicode.org/reports/tr9/#Vertical_Text