Magento – Create Customize Form Validation Messages Source: https://magentocoder.wordpress.com/2011/05/15/magento-create-customize-form-validation-messages/
<--! all the classes http://inchoo.net/magento/out-of-the-box-form-validation-in-magento/ --></-->
<input type="text" id="firstname" name="firstname" value="" class="required-entry"/>
<--! required-entry is the default class to validate de field. You need to crearte a new
class using the same name plus -yourtext -->
<input type="text" id="firstname" name="firstname" value="" class="required-entry-yourtext"/>
<script type='text/javascript'>
//<![CDATA[
Validation.add('required-entry-yourtext', "First name can't be empty!.", function(v) {
return !Validation.get('IsEmpty').test(v);
});
//]]>
</script>