Tutorials and Guides on deploying to GitHub Pages
Tutorials on HTML
This section walks you through step by step.
TL;DR:
Longer Version:
When you use GitHub pages, you can access your website at the url http://username.github.io and the underlying files would be found in your repository, which can be found at url structure http://github.com/username/username.github.io
HTML Specifications https://html.spec.whatwg.org/
CSS Specifications https://www.w3.org/Style/CSS/specs.en.html
Github pages recognizes files using the Jekyll Static Site Generator framework. This is a Ruby language library that converts text (markdown formatted files) into html pages.
Static html pages means that it is an html page that doesn't change when you view it across different timestamps. As in, static is not a script generated page that pulls data from a database (that would be dynamic).
The guides for using Jekyll are easily found through your nearest search engine. Here are several: