Oh and by "mastering css" we just mean to learn the basic box model (width, margin, padding), units (em, rem, px, %, vh, dvh, lvh, etc.), the normal flow of each HTML element, the display basics (inline, block, contents, none, initial, revert, inherit, unset, etc.), positioning (relative, static, absolute, etc.). Flexbox and Grid deserve their own chapters. Ah there's also those capabilities for elements to float and overflow... Also, those are just the basics for positioning we didn't cover things like colors and selectors. Feel free to explore, deep dive and stay up-to date (it's moving fast) so that you can become a real css master!
Maybe it's just a mess that no sane person want to learn. Semantic html is really about creating blog posts, there's not much elements provided beyond that. If you want to create an e-commerce website for example, there is no "card", "carousel" or "popover" element. You have to make them yourself based on existing elements which then makes the css styling confusing.