article by Frank Hellenkamp
Validation of HTML form data
Thanks to the new HTML Standard and the WHATWG Group it is easier than before to have Browsers show standard errors when a user does not provide valid data inside an input element. Unfortunately this only works in newer Browsers, and not in older ones (e.g. Internet Explorer 8).
To make it easier for developers, depage-forms provides an way easy way to add validation to forms that works in all three different places at the same time:
on the server
in the browser, based on the new html standard