JoJo's Bizarre Adventure Book Club (Volume 5) 「最後の波紋」

It appears to be exactly what you said. HTML Standard - the ruby element

<ruby><ruby>東<rt>とう</rt>南<rt>なん</rt></ruby><rt>たつみ</rt></ruby>の方角

とうなんたつみの方角

Therefore, in this example, けん、、みつ、、

As a matter of fact, [Userscript] Forum: IME2Furigana broke just now. It doesn’t account for this scenario.