http://www.w3.org/TR/css-ruby-1/#collapsed-ruby
4.2 Sharing Annotation Space: the ruby-merge property
If the ruby-merge property is to remain, i think that a better name for
the `collapse` value would be `merge`, ie.
ruby-merge: merge;
The meaning of `collapse` is not as obvious as `merge`, to my mind.
From @r12a
This issue is also tracked as Issue 44 in the DoC