* I also don't understand how this is being served. If you go to https://github.com/mathiasbynens there should be a repository called mathiasbynens.github.io right? But I can't find it.
Not exactly - the subdomain is the account and the path name is the repository (and the repository's `gh-pages` branch is what's returned).
(https://github.com/mathiasbynens/rel-noopener)
* I also don't understand how this is being served. If you go to https://github.com/mathiasbynens there should be a repository called mathiasbynens.github.io right? But I can't find it.