This section is informative.
The Ruby Module defines elements for ruby annotation markup as defined in Ruby Annotation [RUBY].
This module adds the ruby element to the Text content set of the
Text Module. XHTML 2.0 supports the maximal content model for the ruby element, defined as follows:
((rb, (rt | (rp, rt, rp))) | (rbc, rtc, rtc?))
As defined in [RUBY], the ruby element is not allowed to nest.