The behaviour of USVString reflection is defined here. jsdom currently implements it as a utility function, reflectURLAttribute(), instead of using webidl2js with [Reflect].

While webidl2js would ideally implement this instead, it requires a URL parser which in turn must have access to the Document and any potential <base> elements, thus creating a dependency on whatwg-url and access to the DOM. These dependencies are probably too far outside the scope of a general purpose Web IDL to JavaScript converter.

However, the README for [Reflect] says that:

In the future we may move this extended attribute out into some sort of plugin, since it is more related to HTML than to Web IDL.

Doing this should allow us to add a USVString reflector without additional weight or complexity in webidl2js itself.