Add WebIDLpedia to that list - so big +1 :)

merge() sound like a good choice as that's what it would do 😀

Trying to handle [Exposed] correctly quickly runs into a problem. When mixins are included into multiple interfaces, the resulting exposure set of the mixin members could be different on the different interfaces. (It's intersected with the host interface's exposure set.) Therefore, a distinct copy of the member will be needed for each interface. Not coincidentally, this matches how Web IDL talks about mixin members:

Each member of M is considered to be a member of each interface I, J, K, … that includes M, as if a copy of each member had been made.

I'm not sure how to copy an instance of Attribute and to change the parent of the copy, if it even matters what this.parent is. One idea inspired by autoParenter is to use a Proxy, but if the proxy is what change the apparent extended attributes, the underlying write() would still get it wrong.

Ideas?

Hmm, so there should be a way to copy a member. For now this kinda works:

// given that we have `attr` of type Attribute
const clone = new attr.constructor({ source: attr.source, tokens: attr.tokens });
Object.assign(clone, attr);