What is a good light two-way data binding JavaScript library?

Might not be what you want, but I like mithril.js.

It's now my go-to solution for HTML-javascript projects. It allows rapid prototyping and simple iteration is a way that most other solutions I've experimented with just don't.

I suggest you play with a few and then choose. (And I do mean play. Make something fun that'll keep your attention).

Not sure if you want to stick with that in the long run. As your project grows, it will grow out of that simple requirement. It's almost always that you need a more mature framework afterwards.