When I was learning the front-end, a wise woman said that I need a solid understanding of HTML, CSS, and JavaScript.
I kept hearing more and more: Frameworks and Library's come and go. I experienced that first hand when moving from Bootstrap 2 to 3, and from 3 to 4. The point is that I was learning a library/framework and not understanding CSS.
I read an article by Cathy Dutton: Stepping away from Sass
I started focusing on CSS more.