In Inner Display Layout Models - ruby, CSS Display should summarize https://drafts.csswg.org/css-ruby-1/#block-ruby and say that

• If the outer display type is block, the element generates a principal block box containing an inline-level ruby container. Otherwise, it only generates a principal ruby container.
• In both cases, the ruby container box establishes a ruby formatting context in addition to integrating its base-level contents into its parent formatting context, which will always be an inline formatting context.