http://dev.w3.org/2006/webapi/WebIDL/#es-attributes says

[[
For each attribute defined on the interface, there MUST exist a corresponding property. If the attribute was declared with the [Unforgeable] extended attribute, then the property exists on every object that implements the interface. Otherwise, it exists on the interface’s interface prototype object or on every object that implements the interface.
]]

Why does it say "or on every object that implements the interface"? That leaves a choice for the implementor and leads to interop problems and harder object detection for authors. It would be better if it required the attributes to be exposed on the interface prototype object.

See http://www.w3.org/Bugs/Public/show_bug.cgi?id=13984 for a case where this causes problems.