You can either trust the individual certificates, or, for more
convenience, you can download their root certificate from their
website and import it into your system/browser/whatever; after you've
done this, everything should work without warnings. Make sure you
compare fingerprints and certificate details when doing this.
You will also need to trust the certificate separately for command
line SVN, which is explained here: http://www.geekforgod.com/2006/12/01/making-svn-trust-a-new-root-ca-certificate/
. Of course, you can alternatively just trust the svn.agavi.org
certificate itself when prompted.
Note to Mac OS users: after importing the root certificate into
Keychain, you might still get warnings at least in Safari. If that's
the case, change the policy for "SSL" to "Always Trust" under "Trust".
Cheers,
David