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It is not clear to me - can I seamlessly compile the same codebase to a TUI and web or does it require web-first design?

I think it requires TUI-first design. Then it should seamlessly compile for web.

I think you're right. The library at the center of this is ratatui - a TUI library for Rust. The web translation layer is new.

Can you write in JavaScript or does it have to be rust?

The primary goal is to serve apps that use https://ratatui.rs to web. You can still bundle Javascript via Trunk though.