But for only one thing i would not use a framework, a library like http://ractivejs.org is enough for that.

That looks pretty similar to Knockout - I don't think you can really claim that one is a library and the other a framework.

Ractive.js just cares about the view - everything else is up to you.

Yes, but the same is true of Knockout - it has no server-side ajax functionality. I'm looking at the code on the example pages and it's solving essentially the same problem as Knockout (there's even a shout out to Knockout on the homepage).

The main difference seems to be that in Knockout has 'observables' that are part of your view model, whereas with Ractive you interact with your view model via Ractive's functions.

Sorry I think of knockout as more of a library than a framework. It's called a framework (and often the smallest/lightest one), but most of the real work is really only done in one or two places