Reviewed-on: https://akkoma.dev/AkkomaGang/akkoma/pulls/132
This allows to format Japanese furigana (aka ruby) notation. Present in XHTML 1.1, HTML 5 and later. Absent in XHTML 1.0, HTML 4 and earlier. See https://www.w3.org/TR/ruby/
This makes it possible to configure their behavior on OTP releases.