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 ;-)
> But that seems pointless.
Making notes for your own consumption?
Upload the file when you are done, perhaps?
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.