That's nice. AFAICT nobody has yet figured out how to run the rust compiler in WebAssembly directly, but it runs in v86. That's why I wondered if they were taking that shortcut for another complex program.

Here's their page about WASM: https://wiki.documentfoundation.org/Development/WASM

It says it uses Qt/VCL. Maybe those can talk to canvas in a fairly straightforward way, or maybe ZetaOffice chose a different backend to get a proof of concept out faster.