I'm having this issue as well. I deleted all digicert certificates from my keychain just in case. Still couldn't get to Github. I can get to the DigiCert Root Certificates download page, but it gives me an invalid certificate warning. It looks like the same issue as Github.

I really, really don't feel comfortable downloading a ROOT CERTIFICATE with an SSL warning on the page. Who knows what could be compromised in this case?

I'm going to try a couple other things first; I'd like to hear from a security expert, should we find this scary or just a small hiccup?