Is it possible to use HTTPS/SSL on GitHub Pages sites with a custom domain?

Custom domains on GitHub Pages do support HTTPS / SSL:

GitHub Pages has supported custom domains since 2009, and sites on the *.github.io domain have supported HTTPS since 2016. 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.

https://blog.github.com/2018-05-01-github-pages-custom-domains-https/

1. Go to https://github.com/**username**/**repo**/settings

2. Check the checkbox "Enforce HTTPS":

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Prior to May 1, 2018, SSL was supported by GitHub Pages only on sites using a *.github.io domain: https://help.github.com/articles/securing-your-github-pages-site-with-https/

It's now possible to use HTTPS on GitHub Pages sites with a custom domain

If you are using CNAME or ALIAS records for your custom domain, you’re all set and your site should be accessible over HTTPS.

If you are using A records, you must update your site’s DNS records with new IP addresses. Please see our guide to setting up your custom domain with Pages and update any A records you might have set.

Once you have HTTPS working:

You can optionally “Enforce HTTPS” for your domain in your repository’s settings, ensuring users who request your site over HTTP are upgraded to HTTPS.

You can read the full announcement here: https://blog.github.com/2018-05-01-github-pages-custom-domains-https/