Sometimes we want to dynamically decide if a form field should be validated or not. Out of the box Prototype Validation does not allow that, instead all form elements are validated.
However, with a simple modification to validator we can skip validating all fields with skip-validation class.
Add your script after js/prototype/validation.js and you are done.
In a non-obstructive way, form validation behaviour can be changed simply by adding class to form fields.