From https://www.w3.org/Bugs/Public/show_bug.cgi?id=29623

https://drafts.csswg.org/css-images-4/#elementsources

reads

partial interface CSS {
  [SameObject] readonly attribute Map elementSources;
};

If we switch CSS over to be IDL namespace instead of interface, it can only use operations, not attributes, as far as I can tell.

elementSources is not implemented anywhere, is it? Should we change it to an operation when switching to IDL namespace? Or should namespaces be able to use attributes in Web IDL?