I think neither library nor framework really fit. A library is something you import, that becomes part of your project, but Svelte's main selling point is that it's compiled away and "magically disappears". In combination with rollup it even does tree shaking for your JS and CSS, anything to reduce code size so that it's as small as possible to embed. The most correct term would probably be compiler, but that would just confuse people :) They'd start comparing it to TypeScript instead of Vue and React then.

I did try Imba out a while ago when I saw Scrimba on Hacker News, looked really neat, especially if you come from a Python background. But it wasn't enough to pull me away from Reagent + ClojureScript :)

With Reagent I really thought that's it, you cannot possibly do better, it removed as much boilerplate as is possible and I strongly believe in the principle that simple things should be simple to do, only add as much complexity as you need, which is what attracted me to Vue before I found Reagent. You can start out with Vue with just four lines of code, you don't need to use templates and components, but you can add as much complexity as you need when your project grows. The problem with ClojureScript + Reagent is that having to use Google's Closure Compiler has it's drawbacks and ClojureScript apps are huge because you've got a huge runtime to bundle with your app, so Svelte 3 has very compelling advantages there.

Aurelia I haven't heard about before, I'll check it out, thanks for mentioning it! :)