How can I master responsive CSS?

Answer 1

Practice makes perfect. You master anything by doing it as much as you can. Make sure that you understand media queries as well as concepts like mobile first design. I find that the best way to master these concepts and really enforce your knowledge and understanding of them is to work on projects that you come up with on your own. If that's not your style, I suggest taking sites that you browse daily and attempt to recreate them. Doing this will not only solidify your knowledge but also see how the sites you frequent daily implement their designs.