It’s important to understand that HTML5 Forms are not meant as your sole means of validation. You still need to do server-side validation and, of course, sanitize the user’s input before you store anything in a database.
In fact, HTML5 form validation is less validation and more, as developer Mounir Lamouri puts it, “like an adviser guiding you in filling the form.” In other words, HTML5 forms provide a quick means of very basic validation to let users know about errors before they submit the form.
tel field actually contains a telephone number.
Lamouri has written a good basic overview of how to use HTML5 forms in Firefox 4, one of the few browsers that supports most of the Forms spec. WebKit browsers are a little behind but still support some of the spec.
Obviously, with very limited browser support at this point, HTML5 forms aren’t ready for prime time. But if you’d like a glimpse of what’s coming, Lamouri’s overview makes a great introduction. If you’d like to experiment with the tools outlined in Lamouri’s post, you’ll need to grab a copy of Firefox 4 beta 7.
For further reading on HTML5 forms, be sure to check out the forms chapter in Mark Pilgrim’s Dive into HTML5, available for both online and offline browsing.