Nim does a lot of things very well, I love writing JSON and parsing JSON in Nim, probably the best experience I've had with JSON, followed shortly by just implementing it as a protocol in Elixir.

Karax's pattern of just using language constructs to assemble HTML isn't really novel, as nice as it is; Ruby has had one for ages in Builder (and several offshoots), Elixir has Temple, and there are probably some in other languages. They're sort of one level of abstraction less than slim/haml, but its quite pleasant writing them. But they suffer some of the same issues Slim/Haml suffer, but also can suffer when trying to use them with component frameworks, or anything that exposes custom tags. This can, of course, be solved via metaprogramming or language-level templates, but it is a concern