Say an X.509 cert is signed by an intermediate cert that's not in your browser. Maybe the root certificate is but not the intermediate cert. At that point it seems like a valid certification path could exist even if your browser doesn't know about it and I guess you'd just be S.O.L?
Seems like maybe there ought to be an extension saying where the issuer cert can be obtained (assuming it's not the root cert) just as how there exist extensions for certificate revocation lists saying where those can be obtained.