If you want to become a good front-end developer, you ought to know more than just ins and outs of CSS. Naturally, I’m talking about JavaScript. You should (if you haven’t already) really get to grips with it.
JavaScript Enlightenment is truly an excellent book that will definitely take your knowledge to a next level. In it, Cody Lindley walks you through all basic concepts and explains in great detail how things work under the hood. Even if you’re a seasoned JavaScript developer, there’s a lot you can learn from this book.