Mastering CSS is not something one can do in a few weeks, or even months. It takes most people years to really build a foundation of techniques and ways to overcome challenges. When you no longer have to wrestle with it, you know you are getting somewhere.
Because of the Living Standard approach being taken with HTML/CSS and ECMAScript the API’s will continue to evolve. Intimate familiarity with the existing recommendations makes it much easier to understand and adopt new ones as they emerge.
Only when we go down that road with some serious intentions do we begin to learn the trade-offs, and where script has advantage over CSS (or the reverse). With the increase in power and speed of modern devices there may be less of a concern either way, but one should be able to reason things out and that takes experience. Lots and lots of experience.