I wouldn't describe TeX as a "standard", but it's pretty hard to argue that it's "closed": TeX is arguably the best documented program in history!
Knuth wrote it using his novel "literate programming" technique in which the source code is embedded in the documentation. Every 4-5 lines of source code has roughly a paragraph of explanation. That source/documentation is published for anyone to read as Knuth's "TeX: The Program" book, accompanied by his "TeXBook", a couple hundred pages each.
That documentation has definitely encouraged all the people reimplementing the TeX layout algorithms, for example MathJax and matplotlib for math layout, as well as a number of alternative tex implementations.
I wouldn't call it easy reading though :) By the way, it's written in Pascal, while all current implementations, afaik, are rewritten in C.
Also, I found "TeXBook" very hard to follow; "The Program" is much more understandable. Every chapter in "TeXBook" starts with a few paragraphs of relatively simply text and then descends into a bunch of additional paragraphs marked with "dangerous turn" signs that talk about things that were never mentioned before. As the author put it, they're explained "somewhere", but it's really hard to find that somewhere.
I don't think that it's a closed standard either. Rather, like Perl 5, it seems to be a non-standardised, or, perhaps less pejoratively, implementation-defined language.
On the other hand, somewhat like Perl, it is also a language intended from its beginning to be extended. Knuth has said that he never expected things like LaTeX to be built on top of TeX; he rather thought that people would hack directly on the TeX source as necessary.
