Reference to BIDI should probably use SpecRef reference to UAX9 (editorial)

[aphillips]

dir member
https://w3c.github.io/manifest/#dir-member

If the base direction is ltr or rtl, override Rule P3 of [BIDI], setting the paragraph embedding level to 0 if the base direction is ltr, or 1 if the base direction is rtl.

The reference to BIDI should probably use the canned definition for UAX9 instead of defining a separate locally maintained copy.

The I18N WG notes that specifying the aspects of UBA here seems like overkill: you are transmitting metadata. It is up to implementations to use direction metadata appropriately.