Take a look at https://sile-typesetter.org/ https://sile-typesetter.org/. IIRC it literally reuses some core C libraries from TeX/LaTeX, and glues them together

Take a look at https://sile-typesetter.org/ https://sile-typesetter.org/. IIRC it literally reuses some core C libraries from TeX/LaTeX, and glues them together in a different way using Lua, keeping Lua as the only language available. Notably, SILE recently achieved the long overdue milestone of math/equations support (https://sile-typesetter.org/2021/09/sile-0-12-0-is-released https://sile-typesetter.org/2021/09/sile-0-12-0-is-released). Obviously, open-source. edit: for some attempt at an explicit discussion of pros & cons & history vs. TeX, see: https://sile-typesetter.org/what-is/#sile-versus-tex https://sile-typesetter.org/what-is/#sile-versus-tex