Earlier quoted context omitted.
On Safari the only option to proceed requires installing the DoD certificate.
I used mobile Safari both on iOS 11 and iOS 10 with no problem on that site. Also on mac OS, at least according to this picture, it's not the CA that's accepted, just the exact site certificate and only for the given site: https://i.stack.imgur.com/vqOBP.png
What I see:
