Yesterday. GitHub Pages going to support for HTTPS.
Today, I remove Cloudflare SSL and change it to GitHub’s.
That’s really simple. First, you need to make Cloudflare working DNS only. Just turn the cloud icon from orange to grey.
Second, disable the custom domain and enable it in GitHub repo settings. Then you will see the message “Enforce HTTPS — Not yet available for your site because the certificate has not finished being issued.” on the last section. Just few minutes later. You can enable “Enforce HTTPS” as you want.
Finally, You can see that your certificate changed from COMODO(Cloudflare) to Let’s Encrypt(GitHub Page).