Was bit confused about what it was at start (thought was running Python client-side). It's basically a web framework ~FastAPI (hence the name, but page rather API-oriented) & Python HTML components* & htmx integration. Got a very popular start[0] but don't know whether it got much traction since (myself didn't knew about it before now).

[0]: https://news.ycombinator.com/item?id=41104305

*Most known similar project to this is Clojure Hiccup.