This is Spglib library ported for the web with LLVM and Emscripten. Follow the tutorial at the Emscripten website to learn details about transpilation of C source code to JavaScript.
Now to determine crystalline symmetry you need only a browser (without any plugins). This is a pure experiment, and to foster any possible ideas and usecases I created an example web-app (validator.html), providing symmetry validation functionality for CIF or POSCAR files. An explicit symmetry information is absent in POSCARs and often omitted in CIFs, so this web-app provides a quick way of checking the symmetry for such cases. No server is used after page loading, so it is possible to drop a file there and check it without Internet connection (and without any concerns about privacy).
I thank Dr. Atsushi Togo-san for Spglib.
See SpglibJS online. More info in the blog: https://blog.tilde.pro/handle-crystalline-symmetry-in-browser-using-pure-javascript-6bb01b99d170