Comment by airstrike

If you count Rust as a "functional language", my personal favorite is iced, which is bloody brilliant and blazing fast https://github.com/iced-rs/iced

It's desktop-first but compiles to WASM

It's honestly "just Rust", so I guess if you're looking for a DSL-type declarative, it doesn't quite fit the bill... but there are design reasons for why it's "just rust" and honestly I much prefer it. It still feels declarative enough