YAPP XSLT is a lexical scanner and recursive descent parser generator, implemented in XSLT. No language extensions or non-standard features are used apart from the nodeset() function. Grammars are expressed in XML form and transformed by the generator stylesheet into another XSLT. A lexical scanner may also be generated from the same grammar.
Emscripten is an open source LLVM to JavaScript compiler. It lets you take code written in C or C++ and run it on the web. It can also be used to compile entire language runtimes, like CPython, letting you run code in those languages on the web as well.