[css-text] i18n-ISSUE-354: Questions about letter-spacing for Arabic script

8.2 Tracking: the letter-spacing property
http://dev.w3.org/csswg/css-text/#letter-spacing-property
Editor’s Draft, 20 March 2014

The length value has the description:

"Specifies additional spacing between visually-perceived characters. 
Values may be negative, but there may be implementation-dependent limits."

For cursive text the spec says:

"If it is able, the UA may apply letter-spacing to cursive scripts by 
translating the total spacing distributed to a run of such letters into 
some form of cursive elongation for that run."

Does this mean that if tracking length is set to 1em for a word that is 
6 letters long, that the total length of the resulting text will be 6em 
plus the letter widths, and that therefore if there are some letters 
that are not allowed to elongate the others will elongate wider than 
1em? Or does it mean that those letters that can stretch will each 
stretch by 1em (possibly resulting in less than 6em overall width)?

If values are negative, does this have any meaning for cursive scripts, 
or is it a hint to use ligatures if any are available (which will result 
in different effects per font)?

And then there are Arabic font styles that don't elongate, such as 
ruq'a. Does the application have to disable letter-spacing if the user 
or the device chooses a ruq'a-style font, or is that the responsibility 
of the author? It seems that it might be hard for authors to signal what 
to do in the case of fallback fonts.



In addition to these questions, I'm also wondering whether the concept 
of letter-spacing as one might use for, say, Latin text really applies 
for Arabic. For example, elongations are often not uniform even across 
the letters that do elongate. (see the example different-elongations.jpg 
and doha-airport.jpg) As currently defined, letter-spacing seems to 
imply some uniformity across letters where tracking is applied.

What may be more useful for cursive scripts like Arabic is the ability 
to specify the overall width of a piece of text, and let the application 
produce elongation according to rules to fill that space. Note that this 
is different from what I described above (6 letters example) in that the 
overall length includes the space taken up by the letters themselves - 
it's more like justification.

Most of the stretching I've seen appears to be more concerned with 
filling a given horizontal space than ending up with an unpredictable 
overall width that is driven by the letters selected.

For example, many signs that include arabic and english make the arabic 
and english the same width, one above the other. The degree of 
stretching is determined by the overall width, rather than just by 
making letters wider.

Even in single-line, non-bilingual situations also it appears to me, 
though I admit I'm not certain at all, as if the desired overall width 
is often what drives the amount of stretching, rather than the desire to 
just have wider characters. See for example the Macdonalds advert attached.