We would like to use some extended attribute types in jsdom, but that webidl2.js cannot currently parse.

interface HTMLMyElement {
  // List of strings (from https://chromium.googlesource.com/chromium/src/+/master/third_party/blink/renderer/bindings/IDLExtendedAttributes.md#reflectonly_a)
  [Reflect, ReflectOnly=("first", "second", "third", "fourth")] attribute DOMString quarter;

  // List of integers
  [Reflect, ReflectRange=(1, 4)] attribute unsigned long span;
};

#222 looked into changing the extended attributes parser to be more restrictive. whatwg/webidl#574 resolved the question by saying that the grammar is intentionally generic. Indeed, the current ExtendedAttribute grammar allows for basically anything given that (quoting from #222):

Any types of brackets must be closed if appears. Commas are allowed only inside brackets (as any comma outside will be a separator for the extended attribute).