Feature request: you might want to let Javascript generate the keypair so that people stop whining about letting someone else generate the private key. Although a valid argument, this isn't going to be used in production environments anyway.... Or is it, guys?!
Feature request: Let people set the key size. Also inform people about the key size (why 4096 is probably overkill at the moment).
Bug: You don't actually mention what the tool does. It generates some sort of certificate, but people will probably be under the impression that it will work instead of spawn another one of these: http://www.page-zone.com/SSL-warning-cpanel.jpg