This is a common fallback technique, but it has inherent limitations.
The iframe element is used as the target of the form's submit operation which means that the server response is written to the iframe.
The following form contains some typical validators needed in a registration form.
This j Query plugin has support for a lot more validation functions, you find them all in the menu to the right.
Browsers that support the XMLHttp Request Level 2 will be able to upload files seamlessly and even get progress updates as the upload proceeds.