When we redesigned two of our sites we put a lot of effort into making the signup process better and focused on making a lot of input validation before posting the page. We also knew that there is a lot of business data that we need to validate before posting the whole signup form, this was achieved with ajax calls to an api. Now we use a jquery validator, implemented as a smarty-plugin, all over the site whenever we have a form with user input. We have found that despite the focus on DOM-manipulation and a lot of graphical plugins, jquery form validation is very powerful combined with the graphical effects you can easily achieve with the library.
See our longer article on Form Validation in general and 10 tips making it work better for you.
Smarty and jQuery should make the perfect couple done right in your front end architecture.