Elm is a good one to get your feet wet too (and develop HTML5 UIs in the process). Because all your Elm code is purely functional, refactoring is quite safe... make your changes and then fix all the compiler errors, and it probably just works. This reduces the penalty for early mistakes.
Elm is awesome for learning functional programming!
I really want an excuse to use Elm in a real-world context. Still haven't needed it, but still excited about the language.
It's good for applications, maybe less so for content-based websites. (Based on your preact post and concern with deployed size.) Give it a try for some internal tools and see what you think.
I've found that where the difficulty lies in Elm is backwards from most UI frameworks. Most of them are easy/moderate to get going, but become increasingly hard to maintain over time. Elm requires a little bit more effort to get started up front (adjusting to FP mainly), but maintenance difficulty doesn't increase much over time. Maintenance is still work, but not particularly risky work.
We use it production, both internal and customer-facing.
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