Section 4.6.20, "The ruby element", says:
In this example, each ideograph in the Japanese text 漢字 is annotated with its kanji reading
The parallelism with "bopomofo reading" and "pinyin reading" in the second and third examples in this section implies that "kanji reading" is being used to mean "reading written in kanji". But in fact, the reading is written in hiragana.
-- Gareth Rees
