A difficult and challenging task I had been given is to use the JQuery validate.js to validate the date fields, but this was then split out into 3 fields. One for day, month and of course year, which sounds easy, but it was not. The issue comes that validate.js is built to validate one field on its own and we wanted to validate not just each field, but also as a whole date.
This libraries purpose is to automatically inject the necessary tags and commands to the users mark-up, so that it meets the standards as much as possible. There are also commands for the users to inject the methods on to specific parts if the library does not get them automatically. Also the automatic functionality can be turned off if only partly needed.
A bit of a small audience for this one, but a project I was put on wanted to us the Galen Framework to test their UI. For people who do not know what this framework is, it is a method of writing acceptance code that matches up to the output in the browser window. This can be coded to work for all different device, sizes and URLs. One hard space I came to was to action the hover state of an element and then test it.