This seems like a good place to plug my library, Solarite.
It's a minimal, compilation-free JavaScript library that adds reactivity to native web components, as well as scoped styles and a few other ease-of-life features.
Reading through the example, it seems like it doesn't do reactivity, as the user code must call render() manually on state changes. Did I miss something?
I'm definitely not the authority on the definition of that word, but in my view I expect reactivity to mean that the UI reacts to state changes "automatically".