Edit the Dhall config (a real CodeMirror editor with Dhall highlighting), then type an alias@domain address and press Run. The actual C pipeline (src/config.c + src/mail.c, compiled to wasm) parses the config, typechecks it, and resolves the address — accept/reject, via alias or catch-all.
The WebAssembly bundle is rebuilt from the C source with make wasm; see the repo for details.