[i18n-activity] Default ruby position may be on the wrong side for tblr (eg. mongolian)

r12a has just created a new issue for 
https://github.com/w3c/i18n-activity:

== Default ruby position may be on the wrong side for tblr (eg. 
mongolian) ==
10.2.39 tts:rubyPosition
https://www.w3.org/TR/2016/WD-ttml2-20161117/#style-attribute-rubyPosition

`before` is mapped in the table to 'left' for 'tblr' writing mode (ie.
 Mongolian).  `before` is also the default value.  This may be 
incorrect for tblr writing systems like Mongolian.

See an example of Mongolian ruby at 
https://github.com/w3c/mlreq/issues/4

Please view or discuss this issue at 
https://github.com/w3c/i18n-activity/issues/334 using your GitHub 
account