Initial proposed changes to AG charter following AC review

To follow up on today's update on the progress of the updated charter:

https://www.w3.org/2019/10/29-ag-minutes#item01

Below are changes to the charter we have suggested to address comments 
from the Advisory Committee. Discussion is ongoing and there could be 
further changes before the charter is finalized. I will send an update 
on further changes as things progress.

The following changes were made to the charter in response to the 
collective comments from the Advisory Committee:

  * Mission: The mission of the Accessibility Guidelines Working Group
    is to develop specifications to support the implementation of
    digital content and applications which aremaking implementations of
    web technologies accessible for people with disabilities, and to
    develop and maintain implementation support materials.
  * Silver scope: Develop Silver (provisional name) to succeed WCAG 2.x.
    The goal is to provide information that can be used to improve the
    accessibility of products when using the guidelines on a variety of
    platforms, without writing normative requirements for thenon-web
    platformsthemselves. As noted in the Requirements for Silver
    <https://w3c.github.io/silver/requirements/>, it will use a
    different framework to allow it to address more disability needs,
    and address publishing requirements and emerging technologies on the
    web such as augmented / virtual reality (AR/VR/XR) and digital
    assistants, and. It will provide non-normative support (for
    instance, through supporting techniques) for authoring tools and
    user agents, and technologies that impact accessibility such as
    assistive technologies, software, and operating systemsauthoring
    tools, user agents, and assistive technologies in order to clarify
    the ways in which web technologies need to work together to support
    people with disabilities. This framework is intended to support
    better coverage across disabilities, and be easier to maintainover
    time, so that the Silver framework will be more durable over time as
    technologies evolve. This will require development of a new
    conformance model, engaging policy makers in that process, and
    testing the model with policy makers from different settings.
  * Silver timeline: Change CR to April 2022 and Rec to April 2023,
    changing note "after charter expiry, subject to change" to "Note:
    this is a projected date after the expiry of this charter and will
    be revised in a subsequent charter based on progess.".

A version of the charter with the proposed changes in place is available 
at https://raw.githack.com/w3c/wcag/charter-2019/charter.html

A diff from the version reviewed by the Advisory Committee is available 
at 
https://github.com/w3c/wcag/compare/9a8c257b5a749fd6bf546ae396341e7d87c39bfe...charter-2019