[css-counter-styles-3] Writing mode based glyphs for disclosure-open/close

Hi,

 From §5.3:
> For the ‘disclosure-open’ and ‘disclosure-closed’ counter styles, the
> marker must be an image or character suitable for indicating the open
> and closed states of a disclosure widget, such as HTML's <details>
> element. If the image is directional, it must respond to the writing
> mode of the element [CSS3-WRITING-MODES], similar to the
> bidi-sensitive images feature of the Images 4 module. For example,
> the ‘disclosure-closed’ style might use the characters U+25B8 BLACK
> RIGHT-POINTING SMALL TRIANGLE (▸) and U+25C2 BLACK LEFT-POINTING
> SMALL TRIANGLE (◂), while the ‘disclosure-open’ style might use the
> character U+25BE BLACK DOWN-POINTING SMALL TRIANGLE (▾).


So with Images 4, it is possible to write a @counter-style rule with 
image symbols that does the right thing based on direction/writing-mode.

Is it possible to do so with characters like U+25B8 ▸ and U+25C2 ◂ 
instead of images? (Using text in a SVG image doesn’t count.)

It’s a bit unfortunate to suggest units there characters if it’s not 
possible to meet the bidi-sensitive requirement that way.

-- 
Simon Sapin