Cannot connect to Linked In on Chrome or Safari on OS-X SSL Issue
I wouldn't start randomly trusting individual certificates for websites. That just sounds like a bad idea. Without the chain of trust to confirm the provenance of the certificates it just seems like too much of a risk.

One option is to visit DigiCert's site and download the specific DigiCert certificates that are missing and install them. You appear to still have the High Assurance EV Root CA certificate installed that the site uses, so you can be confident that you are downloading them from the proper source. Just download the file and open them, Keychain should launch and accept them.

https://www.digicert.com/digicert-root-certificates.htm

I'd also recommend running Keychain First Aid. You can find it on Keychain's application menu.