The HTML5 form validation techniques in this post only work on the front end. Someone could turn off JavaScript and still submit jank data to a form with the tightest JS form validation.To be clear, you should still do validation on the server.

With these JavaScript techniques, the display of server validation errors could be a lot simpler if you expect most of your users to have JS enabled. For example, Rails still encourages you to dump all validation errors at the top of a form, which is lulzy in this age of touchy UX. But you could do that minimal thing with server errors, then rely on HTML5 validation to provide a good user experience for the vast majority of your users.

Therefore client side validation should always be treated as a progressive enhancement to the user experience; all forms should be usable even if client side validation is not present.

It's important to remember that even with these new APIs client side validation does not remove the need for server side validation. Malicious users can easily workaround any client side constraints, and, HTTP requests don't have to originate from a browser.

Since you have to have server side validation anyways, if you simply have your server side code return reasonable error messages and display them to the end user you have a built in fallback for browsers that don't support any form of client side validation.