Magento uses Prototype library to regulate form validation. This comes inside handy, because all you need to do when writing custom form should be to assign a good class names on your input fields, and pass the contour id to VarienForm target.
Let’s look at this example of consent in Magento. I also took a while to extract all validation rules you should utilize, and their error message. You can get the Magento customization methods yourself by looking in js/prototype/validation. Js on brand 414.
You can make this happen from any layout preferably your current module’s layout document, Magento did it into their theme’s layout web page called page. Xml located inside app/design/frontend/base/default/layout/page. Xml.