from point 1 in https://lists.w3.org/Archives/Public/www-style/2015Feb/0235.html :

http://dev.w3.org/csswg/css-ruby/#ruby-layout says "Each ruby
base container is sized and positioned to contain exactly all of
its ruby bases’ margin boxes.". This presumes that the ruby bases
have known positions, which isn't true, since vertical alignment
(particularly for 'top' and 'bottom') can't be done until layout of
the entire line is complete. The simple solution is to say that
vertical-align does not apply to ruby bases (they're always like
'baseline'), and to say that their block-direction size is computed
as for inlines (based on the font, not contents).