Do we know that they didn't have some backend code handing the editing?
I don't think a web where every page is globally editable by default would be a good idea, but I can't imagine at all how it would work without a backend, unless all of the changes are just local. But that seems pointless.
HTTP has PUT and DELETE for a reason ;-)
Being able to change stylesheets, disable or enhance various JavaScript scripts, add notes and annotations, and other things, is exactly the idea of a user agent.
The user makes a request, and then does whatever they like with the answer. Not just whatever is sensible, but whatever they want to do.
If that concept somehow became accepted again... I think the accessible web might well become a solved problem, rather than an endless slog.
Upload the file when you are done, perhaps?
> But that seems pointless.
Making notes for your own consumption?