Unfortunately a form element can’t exist within a web part. The form element already exists in the master page of the web part. To resolve the issue the the jquery file jquery.SharePointFormSubmit.js file can be downloaded from https://sharepointformsubmit.codeplex.com/ to facilitate form submit from within the web part. Instead of the real HTML, use a div and a button with the submit event attached. However the script had to be modified slightly to work according to blog post http://sadomovalex.