Huh, the Microsoft Edge API Catalog lists it as existing in a specification, Safari and Firefox. Since Chrome does not expose a FontFaceSet constructor, the API catalog does not know it is supported, but it is.

Anyway, I believe this is actually specified in the specification, though a bit buried in the WebIDL language.

In https://drafts.csswg.org/css-font-loading/#FontFaceSet-interface you can see -
  // FontFaceSet is Set-like!
  setlike<FontFace>;

In https://heycam.github.io/webidl/#idl-setlike you can see the note that mentions size.

So this is most probably a WontFix, but I will someone more knowledgeable decide.

https://heycam.github.io/webidl/#es-set-size does say that setlike implies a size property. Looking at the generated code for TestInterface2 (has setlike but not size) it looks like the size attribute isn't being added automatically.

jl@, it looks like you added setlike in https://chromium.googlesource.com/chromium/src/+/9016ef32769c396deb32c17dc5f3ff4a6d8acdd4, is this a known issue?

IIUC our setlike/maplike support is limited and isn't fully compliant with the spec. I think a right fix would be improving our setlike/maplike support first (e.g. generate size callback automatically) then removing size attribute from FontFaceSet.

 Bug 697121  includes extending our maplike/setlike support to include size.