The editor for the CSS stylesheet stated that the ruby-position and ruby-align properties were 'unrecognized'

As a note, the editor is not the correct check for whether a property is supported; the list of properties/values supported by the editor differs from the list of properties/values supported by CSS sanitizer.

It does happen to be the case that this property is not supported by CSS sanitizer; this would be identified when you attempted to save.

Would recognition of the property concerned in the editor require a separate ticket, once the sanitizer supports it?

I don't really know. Given how closely tied the two systems are (TStyles and CSSS), I'd say no, but...

The editor uses CodeMirror style rules, it has nothing to do with TemplateStyles. So yes, that would have to be a separate task (possibly upstream).