The HTML5 forms specification introduces a feature that enables you to let the browser handle validation. However, if you use this feature on your website only browsers that support HTML5 validation will validate the form fields; all the other browsers will just ignore the mark-up and allow invalid forms to be submitted.

Microsoft have created a jQuery Plugin called unobtrusive validation which allows you to use standards HTML5 Forms syntax but will then use JavaScript to make the form validation work in browsers that don’t support HTML5 Form validation. An example of how you would use the library can be found here: http://jsfiddle.net/qxsS8/