I don't know what Github uses, but where I work, we do screenshot testing. We have PhantomCSS[0], which is a helper on top of CasperJs that visits pages, takes screenshots and compares them. There are a lot of other tools, Huxley and Selenium come to my mind, that do the same stuff. Even Facebook[1] tests this way.

[0]: https://github.com/Huddle/PhantomCSS [1]: https://www.youtube.com/watch?v=VkTCL6Nqm6Y