[css-ruby] how to specify offset of ruby annotation box from ruby base box?

Is there an expected/intended way to express an offset between a ruby
annotation box and its associated ruby base box in the block progression
dimension?

I see that Antenna House defines an '-ah-ruby-offset' property [1] for this
purpose.

[1] http://antennahouse.com/xslfo/ruby-extension.htm#axf.ruby-offset

The TTWG is trying to determine the best way to express this layout style
property in TTML that can map to CSS, a function which has been requested
[2][3] in order to support SMPTE ST 428-7 [4].

[2]
https://www.smpte.org/sites/default/files/24TB-Captions-Report-IMF-TTML-Features-20121001.pdf
[3] http://www.w3.org/AudioVideo/TT/tracker/issues/228
[4]
http://standards.smpte.org/content/978-1-61482-799-3/st-428-7-2014/SEC1.abstract?sid=db94d6f6-4904-4940-b8f4-cc05b745373e