This website has a certificate with a special X509 extension in it. When you connect to a website with a browser that supports delegated credentials, Cloudflare, instead of using the certificate, uses a special short lived token called a delegated credential to authenticate to your browser.
Using delegated credentials makes it possible to keep the private key for the certificate in special hardware, or on machines with restricted access, without a performance penalty.
This website is more of a technology demonstration. Delegated credentials are available for everyone to use with dedicated certificates on Cloudflare, but currently only Firefox Nightly supports them.