Thousands of bibliography styles, document styles, fonts in multiple formats, as well as basically every macro package ever written. And it's actually several typesetting programs: TeX, LaTeX, PdfLaTeX, XeLaTeX...
I wrote down a bit here: https://news.ycombinator.com/item?id=3264582
Web browsers cannot perform all tasks of TeX, starting with page breaks.
It is even bigger than all of the Web browsers, which can perform all the tasks as Tex and many other once.