Many GitHub repositories don't use GitHub Pages to host their HTML files. GitHub & BitBucket HTML Preview allows you to render those files without cloning or downloading whole repositories. It is a client-side solution and does not involve any third party hosting servers (except for Yahoo! Query Language to fetch assets).
If you try to open raw versions of any HTML, CSS or JS files in a web browser directly from GitHub, all you will see are sources. GitHub forces them to use the "text/plain" content-type, so they cannot be interpreted. This script overrides it by using Yahoo! Query Language.