The rt element is used within a ruby element to place small pronunciation marks next to an East Asian language character.
A ruby element without any rt elements is meaningless, therefore it is considered to be a required element.
On the other hand the rb, rp, and rtc elements are all optional.
- The ruby element is used with East Asian written languages in order to place pronunciation helpers adjacent to unfamiliar characters.
- The rb element is used within a ruby element to mark the base text that is being annotated.
- The rp element is used within a ruby element to place parentheses around pronunciation helpers when true rendering of furigana are not supported by the browser.
- The rtc element is used within a ruby element to surround rp and rt elements when they are used semantically, rather than as a pronunciation aid.