> All that's needed to drastically decrease energy use is to just take late-90s/early 2000s web technology and push it to its limits.

Yes, it doesn't have to be that time frame, but it would be a good reference point. Early 2000s allowed us to do most of what we can now, there are certainly exceptions, but the average website would be no worse. The savings in processing and memory consumption can then either be used to run other things, or extend the usable lifetime of a device. There's no reason why we could not use the same device for 10 or more years, again with some specialised exceptions.

Give that this is specifically a w3.org SIG, I'd suggest doing a LTS web standard, something like 10 - 15 years. Make it have a reduced feature set in terms of Javascript and CSS. For some businesses it would be attractive to know that a solution developed to a specific standard which would mean compatibility across devices and software for 10 years (ideally more, 10 years isn't that long). Newer devices would consume less power and older devices would require less frequent replacement.

The problem is that this would need to find it's way into a browser, which would also need a long term supported and stable operating system, to gain all the benefits.