[css-fonts] Named instances and collections

[litherum]

Named instances and collections are conceptually similar: they both allow a single file to hold a finite set of faces, where each face has a name. However, in CSS they are currently triggered using wildly different mechanisms:

For font collections, you say:

@font-face {
    font-family: AMemberOfACollection;
    src: url("aFontCollection.ttc#TheMemberIWant");
}

For named instances, you say:

@font-face {
    font-family: ANamedInstance;
    src: url("aVariableFont.ttf");
    font-named-instance: TheNamedInstanceIWant;
}

I think we have 3-4 options:

1. Keep it like it is
2. Allow named instances to be specified in the fragment of the src URL
   2b. Delete font-named-instance entirely since it's unimplemented, and say the only way to use named instances is in the fragment of the src URL
3. Allow the postscript name of a member in a collection to be supplied to font-named-instance

[litherum]

Thinking about this, we may actually have accidentally implemented option 2 in the Apple ports of WebKit, just because of how platform APIs expose named instances.

[litherum]

My natural thought would be to do option 2 and 3 (but not 2b). And, if content uses a URL fragment but not font-named-instance, expose the URL fragment in the CSSOM and the CSS Font Loading API under the font-named-instance key. And if an author supplies both, then font-named-instance wins.

[litherum]

@svgeesus @drott @jfkthame