An HTML5 form validation plugin for jQuery. Works on all major browsers, both new and old. Implements inline, realtime validation best practices (based on surveys and usability studies). Developed for production use in e-commerce. Currently in production with millions of users.
There are stubs in h5Validate to add error messages, similar to these: http://sandbox.leigeber.com/messages/messages.html
However, they need to be implemented so that they are easier for users to hook up and trigger.