You're supposed to have installed the client SSL certificate in your browser before visiting that URL. It caught me out too initially. They use client side SSL certificates for authentication. I don't know any other site which does this.
It's the first time I came across client side certificates as well. As others have pointed out you have to jump through a bunch of hoops to get a certificate from StartSSL, but if your free time is cheaper than a certificate from another party it can still be worth it (especially because the cost repeats with other parties). I've been a happy customer for more than a year now.
