When a site asks for a client certificate, not all certificates are presented.
At www.pkiuniversity.com/sandbox/index.php, I am asked for a client certificate. I get to choose from a list of the certificates issued by startcom but not my own. The extended key usage does mark it for client authentication. The root certificate corresponding to the signing private key is also in the store. Why don't these certificates pop up. They do in Safari.
Opaite Mbohovái (1)
If you're interested, I get my certificate from reloid.com/enrollments/cheapcerts3/getcert.php?email=me@fred.com This is designed to be a very insecure certificate with no chance of being added to the built-in cache.