If your browser says it cannot display the page, then it is not accepting the certificate presented by the Registrar Web server. You must accept the server certificate before a secure connection can be established.
If your certificate is valid for WLCG Registration use then you will see a page telling you this and you can proceed with the registration.
If your certificate cannot be used, check if any of the reasons below apply to you.
You have not loaded the certificate into your browser.
The certificate is not issued by one of the list of accepted Certification Authorities.
The certificate validity date has passed.
The certificate has been revoked by the Certification Authority