Mithril is conceptually very close to React (it was released a bit after, but developed independently), and it comes with less cultural baggage.
It can be used productively with plain ES5, thought it also supports JSX and ES6 constructs (or even TypeScript) if you like it. I'd recommend TypeScript for large apps, actually.
The API is also closer to the plain DOM (e.g. no synthetic events). More advanced uses of the lib will lead to learn common Web standards (JS/DOM) rather than framework-specific idioms.
It can be used productively with plain ES5, thought it also supports JSX and ES6 constructs (or even TypeScript) if you like it. I'd recommend TypeScript for large apps, actually.
The API is also closer to the plain DOM (e.g. no synthetic events). More advanced uses of the lib will lead to learn common Web standards (JS/DOM) rather than framework-specific idioms.