Ruby Markup Content Model: if a ruby annotation range consists of an rtc element that contains no rt elements, the single ruby annotation unit represented by its contents spans (is paired with) every ruby base unit in the ruby segment., 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-412.html