Parker Moore, on GitHub's blog:
Today, custom domains on GitHub Pages are gaining support for HTTPS as well, meaning over a million GitHub Pages sites will be served over HTTPS.
We have partnered with the certificate authority Let’s Encrypt on this project. As supporters of Let’s Encrypt’s mission to make the web more secure for everyone, we’ve officially become Silver-level sponsors of the initiative.
If your custom domain uses
ALIAS records, no action is required to go HTTPS. If (like me), you have a custom domain using
A records, follow along here.