Looks like a wasm compilation of Devilution, which is a decompiled / reverse engineered version of the original game (they left debug symbols in in some releases apparently).

It definitely violates their copyright if you were wondering. I doubt they care though.

The shareware .mpq is linked from a github page which corresponds to this repository: https://github.com/d07riv/diabloweb/

Which appears to reference source code here: https://github.com/d07RiV/devilution

Which appears to be emscripten based on the build scripts: https://github.com/d07RiV/devilution/blob/master/build.bat