Joomla has client-side and server-side form validation capabilities.
.. is done after having submitted the form and will normally return to the form when not validated with some extra messages. It used the HTML attributes required ("true" or "required") and validate (with value being a joomla or custom rule; e.g. validate="email") This is the minimum requirement, since everything before that can be overridden on the user side.
More here: Server-side form validation