[css-font-loading] [info discovery] Named instances should be discoverable too

[litherum]

Currently, there are no facilities to expose named instances. At least being able to list them would seem valuable, as there's a font-named-instance descriptor which accepts these strings. We may want to go farther and actually describe their definitions, too.

[litherum]

I guess we could start with

interface FontFaceNamedInstance {
    readonly attribute DOMString name;
    /* Maybe more here in the future. For now, just the name. */
};

interface FontFaceNamedInstances {
    readonly setlike<FontFaceNamedInstance>;
};

partial interface FontFaceVariations {
    readonly attribute FontFaceNamedInstances namedInstances;
};

If the strings are localized (I think they are but I'm not sure), we'd have to deal with #6595 as well.

[litherum]

Thinking about #6600, we may also want to expose information about members of a collection. There isn't much philosophically different between named instances and members in a collection (aside from the mechanisms they use internally).

[litherum]

Maybe what we actually want is something like

interface FontFaceNamedInstance {
    readonly attribute DOMString name;
    /* Maybe more here in the future. For now, just the name. */
};

interface FontFaceNamedInstances {
    readonly setlike<FontFaceNamedInstance>;
};

interface FontFaceCollectionMembers {
    maplike<DOMString, FontFace> members;
};

interface FontFaceCollection {
    /* It's expected that only one of these will have contents */
    readonly attribute FontFaceNamedInstances namedInstances;
    readonly attribute FontFaceCollectionMembers collectionMembers;
};

partial interface FontFace {
    readonly attribute FontFaceCollection collection;
};

Or maybe even something like this instead:

interface FontFaceMembers {
    setlike<DOMString>;
};

interface FontFaceCollection {
    /* It's expected that only one of these will have contents */
    readonly attribute FontFaceMembers namedInstances;
    readonly attribute FontFaceMembers collectionMembers;
};

partial interface FontFace {
    readonly attribute FontFaceCollection collection;
};

Or maybe even just:

partial interface FontFace {
    // Returns the union of collection member names and named instances names
    readonly sequence<DOMString> collectionMembers();
};

... Or maybe even reuse FontFaceSet?

partial interface FontFace {
    readonly attribute FontFaceSet collection;
}