External records allow sub-records, so it does make sense that you can create a record with other records inside it. NFC doesn't define any limit of sub records with sub records, but doing more than one or two would be quite rare.

If web idl doesn't allow this, then I don't know how to spec it

If we need to break the structural definition of recursive data, we can define a function in NDEFRecord to init its data from an NDEFMessageInit. A breaking change :).

Maybe @littledan or @domenic have ideas here

Prior to #454 the data field used the any type instead. This violates the spirit of the WebIDL specification but would give us the opportunity to add a depth limit to the create NDEF message steps because we get to define when and how that arbitrary value is interpreted as an NDEFMessageInit.

Yes, using any or object and doing the conversion in prose with an additional depth-check step seems like a good solution.

My involvement with this spec is very minimal, so it'd be great if someone could make the required changes here. I can help with the implementation changes later.

I'll work on a spec PR in the following days.

I've started a spec PR at #621. Please let me know what you think.

@rakuco Spec has been updated. See https://w3c.github.io/web-nfc/#creating-ndef-message and #621

Thank you!