To Display a label if field value doesn't exists on submit the form we can use mootool validation as follows.
//Example Form Field
<input type="text" size="30" class="required invalid" value="" id="jform_contact_name" name="jform[contact_name]" aria-required="true" required="required">
<span style="display:none;color:#993300;margin-left: 111px;" id="jform_contact_name-lbl_id">Enter Name</span>
//Example Form Field
<input type="text" size="30" class="required invalid" value="" id="jform_contact_name" name="jform[contact_name]" aria-required="true" required="required">
<span style="display:none;color:#993300;margin-left: 111px;" id="jform_contact_name-lbl_id">Enter Name</span>