There is a discussion of this in this blog post, as things changed with 5.7.3: https://www.percona.com/blog/2017/06/27/ssl-connections-in-mysql-5-7/
The link in that post, though, has changed as I think the documentation may have been updated. This is a better page from the core manual to view (referenced from the CREATE USER syntax page): https://dev.mysql.com/doc/refman/5.7/en/encrypted-connections.html. From the manual:
As of MySQL 5.7.3, a client need specify only the --ssl option to obtain an
encrypted connection. The connection attempt fails if an encrypted connection
cannot be established. Before MySQL 5.7.3, the client must specify either the
--ssl-ca option, or all three of the --ssl-ca, --ssl-key, and --ssl-cert options.
As you can see, before 5.7.3 the client must specify either --ssl-ca OR all three. There is a full discussion in the MySQL documentation.
Disclosure: I work for Percona