jQuery & CSS
The biggest difficulty for any full stack developer comes when they must challenge the mathematical side of their brain to become artistic, and vice versa. A front end developer(CSS, jQuery, UI, Graphic Design) is often comes from an artistic background. A back-end developer(aka web developer) is expected to be able to quantify, with education or experience, a computer science and/or math background.
“CSS is focused on beautification; whereas, jQuery is focused on functionality and PHP is the controller that manipulates data.” ~ Source: A Full Stack Developer