Re: [w3ctag/design-reviews] Question: How should `Document`-scoped objects interact with `window.navigator`? (Issue #1231)

christianliebel left a comment (w3ctag/design-reviews#1231)

@domfarolino Thanks for reaching out. We agree with you that if an API is effectively per-Document, it should live on `document`.

The TAG has been discussing a new design principle (https://github.com/w3ctag/design-principles/issues/448) to provide guidance on this question. Our response draws on that discussion and on the [HTML Standard](https://html.spec.whatwg.org/multipage/system-state.html#the-navigator-object), which states:
> Instances of Navigator represent the identity and state of the user agent (the client).

The model context does not fall into that category. Furthermore, the `navigator`-based alternative either pushes complexity onto implementations or conceals the document-scoped nature of the API behind reset logic. That doesn't seem worth it, especially given that a new `Document` object is created on every navigation anyway.

We'd therefore recommend `document.modelContext`. Should WebMCP later gain worker support, a worker-only `self.modelContext` could be added. There's also precedent for this pattern: the `FontFaceSet` API is exposed as `document.fonts` and as `self.fonts` in workers (see [The `FontFaceSource` mixin](https://drafts.csswg.org/css-font-loading/#font-face-source)).

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