I'm skimming the linked Web Sustainability Guidelines. It's pretty much the normal stuff HN-types have been banging on about in every thread on webdev for the last decade or two. I don't really see how this will change anything.
I looked to see what those guidelines had. It has nothing about pages actually having contents if JavaScript isn't ran or CSS isn't supported.
Now it can carry a weight similar to WCAG levels, which means that product managers and customers might pay more attention to these requirements, especially if they like ticking boxes.
"Our new update means we reach WGAC 2 Level AA to > 90% and WSG to 60%, next release we aim to reach WSG to >70%" might be something we hear next year.