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 using a CORS proxy to fetch assets.
If you try to open raw version of any HTML, CSS or JS file in a web browser directly from GitHub, all you will see is a source code. GitHub forces them to use the "text/plain" content-type, so they cannot be interpreted. This script overrides it by using a CORS proxy.
What it does is: load HTML using CORS proxy, then process all links, frames, scripts and styles, and load each of them using CORS proxy, so they can be evaluated by the browser.