Validating your form? Nope!
Doing styling work? *buzz* WRONG!
You want to make your content do some amazing things stylistically? You go find an awesome jQuery plugin that allows you to do some crazy shit really easily without having to think of how to do it in CSS. I am sure you have done this because so have I. The problem with this approach is that it really makes it harder to understand all of your JS code (if you have a lot of them) when you start to add styling stuff in there as well. Things become harder to track and maintain and styling updates become a pain-in-the-buttocks.
I personally think that all style manipulation work is the job of CSS. I mean the damn name has the word Style in it. Which is why, in the recent years, many CSS libraries are popping up to replace jQuery plugins that do fancy stuff.