I'm interested in determining the steps needed to create a 'sustainable' startup in web design, which should be somewhat different from graphic design firms. Topics include computing energy cost of web pages, "green" web hosts, sourcing for web design, efficient programming practices - you name it. All in pursuit of sustainability, which might be documented in a "sustainability" deliverable for a web design firm.
Have you thought about looking at sustainability from the perspective of actual development costs of web design? Thinking about a client's needs in the long term and completing technical work formed around the basis of this thought process will allow you to produce solutions that are sustainable both environmentally and for the client over extensive periods of time.
Many businesses re-design their websites multiple times and/or have enormous development costs that could be avoided through correct requirements elicitation and "future proof", modular designs.
This goes across all areas of software development, but if you're not looking for quick money or charging outrageous maintenance and support fees, effort to drive forward "sustainable" web design may be better placed within the code that drives your hardware, rather than the other way round.
In web design your main factors will be energy.
Here is a link of some general energy consumption estimates
Probably a lot of calculators out there for energy consumption. But for a sustainability document you probably want more than just consumption, you would want what you offer to generate energy or put things back ... planting trees, using windmills, have everyone work on a bicycle that powers their laptop or something;)