Rather than a working group (WG), a Sustainable Web Interest Group (IG) with open participation would better enable the Web Sustainability Guidlines (WSG) to have a larger impact sooner, with broader support.
Proposal: rewrite the current proposed SustyWeb charter as charter for a SustyWeb IG and provide a plan for publishing the WSG as a Note, rapidly iterating similar to how the Community Group is already iterating on the WSG Report, with the eventual goal of publishing an AC-approved W3C Statement to give it more formal standing.
An IG would have less process than a WG. It would for example avoid things like patent-related procedures, disclosures, etc. which should be unnecessary for the WSG.
The IG charter could also define custom success criteria for a WSG Statement that better reflects the varied needs of providing a broad spectrum of sustainability guidelines. A broad set of sustainability guidelines would better achieve sustainability goals than subsetting or restricting the guidelines to only those that pass a more precisely objective testability bar that is expected for purely technical specifications for interoperable independent implementations.
At a high level the WSG is also more similar to the Ethical Web principles, which itself is destined (eventually) for a W3C Statement.
cc: @ianbjacobs, @caribouW3