[MPEG-OTSPEC] CSS context for metrics request

[fantasai]

CSSWG requested additional metrics for the vertical bounds of writing systems 
other than Latin/CJK/Indic in its liaison statement from January 2020:
 
https://lists.w3.org/Archives/Public/www-archive/2020Feb/att-0005/CSS-SC29-20200113.pdf
I wanted to provide some background for this request.

The context for this request is the CSS Inline Layout specification which is 
currently under development.
Official draft: https://www.w3.org/TR/css-inline-3/    # latest WG-approved
Editor's draft: https://drafts.csswg.org/css-inline-3/ # tip of tree
Of particular interest would be the 'text-edge', 'leading-trim', and 
'initial-letter-align' properties: which control the spacing of text with 
respect to adjacent content; and the sizing and positioning of 
drop/raised/sunken caps or their equivalent in other writing systems. These 
depend on such font metrics.

The corresponding CSSWG issue is
   https://github.com/w3c/csswg-drafts/issues/5244

For a video introduction to the CSS inline layout model and some of the 
problems we're trying to fix with it, see 
https://www.youtube.com/watch?v=OtlGo48iTOk

Also, Ethan Wang from Microsoft has written a blog post explaining the 
motivation for leading-trim, which might be useful (particularly as it has 
good illustrations):
 
https://medium.com/microsoft-design/leading-trim-the-future-of-digital-typesetting-d082d84b202

Fwiw, feedback on this specification is quite welcome... I imagine this group 
in particular would have useful insights on these sections which deal directly 
with font metrics, and which are a bit shaky atm:
   https://www.w3.org/TR/css-inline-3/#css-metrics
   https://www.w3.org/TR/css-inline-3/#baseline-synthesis-fonts

Issues can be filed in the CSSWG repo at
   https://github.com/w3c/csswg-drafts/issues
-> or, if that is troublesome, by sending a message to the archived mailing 
list www-style at w3.org https://lists.w3.org/Archives/Public/www-style/
with "[css-inline-3]" as part of the subject line. (I can also take feedback 
directly myself, as the editor, if that's easier.)

Also feel free to ask questions about the spec, I'm happy to answer either 
here or on some other channel as you like.

Thanks for your consideration~
~fantasai