A.1 Supporting Ruby Layout
https://drafts.csswg.org/css-ruby-1/#default-ua-ruby

the default UA style sheet information includes the following:

rtc:lang(zh), rt:lang(zh) {
  ruby-align: center; }

This establishes centred display for ruby for Chinese, but there are other language subtags that indicate that text is chinese besides the generic zh. These should presumably also be included here.

The current list of more specific tags recommended by the IANA subtag registry rather than zh is:
cdo cjy cmn cpx czh czo gan hak hsn lzh mnp nan wuu yue.