We use the Dreamweaver Bindings panel to populate the choices available for Server Validations.
this is an issue in the Dreamweaver bindings panel not recognizing HTML 5 element types.
For the time being, you will need to change the type to text, then change it back after using the server validation.
NOTE: in the curent version of Data Bridge, we use jquery for client validation. we use the HTML5Validate script which will allow for you to use standard elemnts types and emulate HTML 5 validation across browser platforms