There are many programs that implement subsets of LaTeX/TeX. For example, for math layout I believe Mathjax and matplotlib have both essentially copied Knuth's

There are many programs that implement subsets of LaTeX/TeX. For example, for math layout I believe Mathjax and matplotlib have both essentially copied Knuth's program, based on his "TeXBook" and "TeX: The Program" books, which document the TeX code extremely thoroughly using his "literate programming" technique. TeX is one of the best documented programs in existence. Google "Latex implementation" and you will see a lot of hits. I see a Java implementation, Windows implementations, LaTeX3 and LuaTeX are referred to as reimplemetations 'nearly from scratch', etc.