Specifies that a value must be entered in this weblet before the form can be submitted. . Note that, at the time of writing, Safari and Internet Explorer do not support this property.
Note: HTML 5 form validation can be turned off by adding a novalidate attribute to the <form> tag or submit button (this is automatically done by setting validationMethod to 'none' on the standard shipped layouts). In addition, different browsers have differing levels of support for HTML 5 validation and there are many ways for malicious users to bypass client-side validation. So, while client-side validation can improve the user experience, you should never rely on it. Always back it up with server-side validation.
True, False, or any valid XPath expression that returns a boolean value.