As someone who doesn't follow the standardization process, I would love an explanation how proposals like this fail even when they have widespread support and no downsides. Is there an easy answer to this or is it different from case to case?
From what I've seen, standards need to be picked up by browser vendors for them to become established.
Usually one vendor will implement it behind a flag, and if it gains traction then other vendors will follow suit before it becomes a generally available feature.
Personally, I don't like the use of indexing to determine the JSON structure (like this `<input name="foo[0][1]">`). I think it's way more powerful to derive the JSON structure from the DOM tree. That's why I wrote some code to do exactly that: https://gist.github.com/mjepronk/5b33eaa90ecf11a5c2c47935a2c...
The use of `<fieldset>` and `<section>` may not be the best choice. You could use `data-` attributes on arbitrary tags for instance.
Looks like supporting an enctype of application/json was proposed, but never went anywhere.