Bopomofo ruby on the right in horizontal text is the norm, not just frequent

[r12a]

2.1. The ruby element > Example 3
https://www.w3.org/TR/html-ruby-extensions/#ruby-bopomofo-ex

In horizontal writing, they are frequently also typeset to the right, in this case sandwiched between individual base characters

change to

In horizontal writing, they are usually also typeset to the right, in this case sandwiched between individual base characters:

Placing bopomofo over the base in horizontal writing is not typical. The norm is to appear to the right of the base.

[r12a]

@xfq adds:

Indeed. See also https://www.w3.org/TR/clreq/#h-positioning_zhuyin_symbols