Their certificate is signed by a trusted CA, but they forgot to ship the intermediate certificate.

I always test my domains with Quals SSL server test:

https://www.ssllabs.com/ssltest/analyze.html?d=staticman.net

There are a couple of other issues with their SSL setup. I also recommend Mozilla's SSL config generator:

https://mozilla.github.io/server-side-tls/ssl-config-generat...