Comment by pavpanchekha

Author here, and I also teach web dev, including CSS, at the University of Utah (including this semester). Newer parts of CSS, like flex-box layout are both simple and powerful. Just use those! I think it's important to start thinking about learning all of the Web Platform like you'd think about learning all of the Windows APIs or all of the Linux system calls or all of your favorite programming language's features. People rarely do! (I have 15 years of Python experience, and I do not understand metaclasses or async.) There are lots of weird obscure corners, but you don't need to know those to build websites.