This is super cool! It seems that endb uses emscripten to compile to WebAssembly, I wonder if they tried WASI or WASIX (https://wasix.org) to target both browser and server environments!
We did try to get WASI to work, but ECL itself doesn't yet target it. It fell down on missing set/longjmp support in wasi-libc. We haven't tried WASIX yet.
There are other approaches than the one we've taken so far, and we're still experimenting. But for the current demo, we're quite happy with Emscripten. The entire Wasm ecosystem feels pretty darn magical. :)