[webvtt] Issue: Consider adding 'ruby-align' as a supported CSS property marked as i18n

r12a has just labeled an issue for https://github.com/w3c/webvtt as 
"i18n":

== Consider adding 'ruby-align' as a supported CSS property ==
https://w3c.github.io/webvtt/#selectordef-cue

> :​:​cue

In https://www.w3.org/Bugs/Public/show_bug.cgi?id=28183 it is pointed 
out that 'ruby-align' exists and could be useful. It's supported in 
Gecko and IE.

We have an `align` cue setting which sets 'text-align' on the root 
element, and we don't allow setting 'text-align' with `::cue` or 
`::cue()`, but I don't see any problem with allowing 'ruby-align'. It 
doesn't affect the size of the ruby box, and you might want different 
alignment for the whole cue vs. ruby. For instance, if a cue contains 
a single ruby annotation and the cue is start-aligned, it could make 
sense to start-align the ruby as well. But if a cue contains a normal 
run of text and has some ruby annotations here and there, it might 
make more sense to use the default `space-around` ruby alignment 
regardless of the cue's alignment.

See https://github.com/w3c/webvtt/issues/265