If you're new to XSS, we highly recommend checking out our interactive labs. They're a great introduction to the topic - covering different vulnerability varieties and how to both catch and prevent them.
Looking for something a bit more advanced? PortSwigger Research has a wide variety of cutting-edge posts covering XSS.
Hone your techniques and pick up some advanced skills with our full list of XSS research below. Covering everything from XSS in hidden input fields to performing CSS powered timing attacks by abusing Jquery.
Make sure to get acquainted with our cheat sheet too - it's consistently updated with new vectors from us, as well as some insightful community contributions.