tts:rubyReserve + tts:lineHeight (issue #779)

Hi Cyril, Glenn et al.,

The following is based on my working on implementing tts:rubyReserve in
imscJS.

tts:rubyReserve is intended to reserve room to contain inline areas
generated by ruby annotations and emphasis marks, regardless of whether the
latter are present at a given instant in time.

As currently specified, the amount of room reserved is explicitly specified
by the author as a <length>, or, if left unspecified, set to a default
value equal to 50% of the used value of the tts:lineHeight applicable to
the p element.

I see a fundamental challenges with this approach:

(a) neither tts:ruby nor tts:textEmphasis specify the amount of room needed
for ruby annotations and emphasis marks, respectively -- the amount of room
is left to the implementation. It is therefore impossible for the author to
accurately specify a <length>, and the default value might not be
appropriate.

(b) the default value is not sufficient if the author specifies a font size
for ruby annotations that is different from the ruby base font size.

I suggest exploring the following tweak to the semantics of tts:rubyReserve:

"""
The implementation reserves room for ruby annotations and emphasis marks
assuming:
- the font size of the base text is the same as that of the <p>
- the font size of the annotation/emphasis mark is specified by the
<length> or, if the <length> is not specified, computed according
to 10.2.21.1
"""

I will be on the road and unavailable for the next two TTWG calls. I am
however available today and tomorrow to discuss live​, and will be
available over email.

Best,

-- Pierre