As a developer of a browser application, I would presume that either the plug-in, the accessibility application, the content provider or the operating system software should be providing most, if not all, of the requirements. At this juncture, I would suspect that there will be less adoption of this specification then is desired. As mentioned above, clear, concise requirements must be given. If the requirements were stated in such a fashion that providing a mechanism to access the necessary data, would also aid is quicker adoption of the specification. For example, if the specification stated that the browser infrastructure must support a mechanism to allow the user to set 'whatever,' then the browser vendors could provide the underlying support and then different chromes could be provided based on the need of the user.
Users have specific likes and dislikes in regards to software applications.
Platforms also play a major role in determining what secondary applications can be used. That approach seems to be more easily adoptable. What is timely to me, may be unacceptable to someone else. Be specific, give a time range that is deemed acceptable. Visual orientation will be costly in processing the data based on resolution, window size, etc.