Answer by Leandro Meili for Linking to another html page on Github

Your should use absolute paths:https://<user.name>.github.io/<repo.name>/

If your index and about.html is under the root folder then you should type:

<a href="https://<user.name>.github.io/<repo.name>/about.html" title="About Me">About Me</a>

For navigating folders like /css or /js, make sure the folder is under the root.

Example link would be:
https://<user.name>.github.io/<repo.name>/css/style.css