Various free CDN providers (e.g., cdnjs.com, rawgit.com) host copies of MathJax, see http://docs.mathjax.org/en/latest/configuration.html#loading-cdn for more information.

The MathJax Consortium no longer provides CDN hosting at cdn.mathjax.org, see https://www.mathjax.org/cdn-shutting-down/ for more information.