I can still remember the horrors of page state. The server would keep track of what the client has and only send HTML fragments to the client. Early-days ASP, Prado and the likes did this and it was a terrible idea. HTMX sounds very much like that, but the packaging is nicer. Ultimately, the problem is that sometimes you need to update more than just the tiny, well-defined part that is the todo list and several parts of the UI need to change when a request is made. By which I mean this happens all the time. The road to hell is paved with todo list implementations as a proof that a system works and is good. Please show me a moderately complex login system with interlinking interfaces implemented in this.
Unpoly and Hotwire are somewhat similar to HTMX, but they update entire sections of a page (i.e., "page fragments") as you describe. They perform a DOM diff and update everything wrapped inside an HTML tag (e.g., a <div>) with a special attribute. Unpoly does this within the app's normal request/response cycle, while Hotwire uses websockets to stream updates.
