The IDL for GPUDeviceDescriptor includes an invalid default value for requiredLimits member. It specifies the default value for requiredLimits to be {}, however, {} is only valid for "dictionary type, or a union type that has a dictionary type in its flattened member types" (https://webidl.spec.whatwg.org).

As far as I can tell, there is unfortunately no way to specify a default value for an optional record in an IDL dictionary, and would probably have to be specified in prose.