Summary:
`ruby-position: alternate` is the new initial value of the `ruby-position` property, it allows ruby text container (rtc) to alternate automatically on line's side without needing to specify a different value for each rtc. This is a more useful value when multiple rtcs are present.
Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1694748
Standard:
https://drafts.csswg.org/css-ruby-1/#rubypos
Platform coverage: all
Preference: layout.css.ruby.position-alternate.enabled
But I intend to turn it on by default on landing as this is a relatively minor feature. It's put behind a flag only because it changes existing behavior and it would be better to be able to rollback if any issue arises, though I don't expect that to happen.
DevTools bug: n/a, just a new value for an existing property, devtools' css properties db will be updated accordingly
Other browsers: no signal, other browsers are currently behind on ruby implementation, and don't support multi-level annotations
web-platform-tests: new test is added