2.2.4 [W3C-RUBY] Section 2.6, The rt element
The specification states:
The rt element is the markup for ruby text. For simple ruby markup, only one rt element may appear. For complex ruby markup, multiple rt elements may appear inside an rtc element, and each rt element contains the ruby text for the relevant base text, represented by the corresponding rb element.
All Document Modes (All Versions)
All rt elements within simple or complex ruby elements are displayed.