So like, Obsidian, but for HTML?

The file itself provides both the dynamic functionality and data storage, but you need an engine (like obsidian) to make the data persistence and dynamic parts work together. I.e. if there is a button that adds a task to a todo app, your engine modifies the HTML file with the new content.

I finally also recalled this project (tiddlywiki), but CMIIW, the statefulness of the hyperclay is only for developers, the end user will get the same conventional html. Without some kind of common solution/protocol/standard on the browser side that would allow persistence it's not so exciting. Theoretically there might be some kind of simple protocol that would save html file versions on the server based on the cookie, but there are so many ways this can go wrong