Why does the text shadow direction depend on the writing direction?

Hi all,

The direction of the shadow offsets specified by tts:textShadow currently
depends on the writing direction [1]:

"""
The first <length> term denotes the offset in the inline progression
direction of the associated area [...], the second <length> term denotes
the offset in the block progression direction of the associated area
"""

Is there a specific reason for this divergence from CSS, where offsets are
relative to the horizontal and the vertical? I would think that the shadow
direction should remain consistent for all text on the root container,
regardless of the writing direction. For example, if lrtb and tbrl texts
are mixed on a page, wouldn't it be weird if the shadow were different on
both texts, as if the light source were not uniform?

Best,

-- Pierre

[1] https://www.w3.org/TR/ttml2/#style-value-shadow