HTTP Public Key Pinning (HPKP). This page is at 1d.pw. Web server just sent key-pin HTTP header to your browser. That key-pin covers subdomains, so if you now go to pin.1d.pw - you should get security error page from your browser (if it supports HPKP).
HTTP Public Key Pinning (HPKP) - is a security extension for HTTP.