Ruby Markup Content Model: some explicit and implicit base units, one implicit rtc as the first level annotation, an explicit rtc with some explicit and implicit annotation units as the second level, and a spanning annotation as the third level, adding rp in all the places where it's allowed by the content model, as well as some where it's not.
third_party/blink/web_tests/external/wpt/html-ruby-extensions/html-ruby-603.html