I think contenteditable is more akin to a rich-text document, while Hyperclay goes a bit beyond by allowing JS to edit the DOM too. I think Smalltalk images and virtual machines are a closer comparison, but applied to the web. You download the image, with some running code, use it, and persist the whole application state.

Yeah, sounds like Tiddly Wiki or similar except with the need for a backend component.

Edit: actually it looks more like a library/framework to make such apps. And now I'm not sure if it needs a backend component or nodejs or not.