The unobtrusive JavaScript framework for server-side web applications. Unpoly enables fast and flexible frontends while keeping rendering logic on the server. It has no dependencies and plays nice with existing code.

Source: Unpoly: Unobtrusive JavaScript framework

Reminds me of parts of HTMX and Turbo (Hotwire).

Kind of thematically similar, but no longer maintained: Behaviors.js: Define dynamic page behaviors via HTML attributes