This is for people who have never written HTML or CSS and want a page that looks right when they open it. You will write real markup, style it with CSS, open it in your browser, and stop pasting code you don't understand. If you skip the basics and copy snippets until something looks fine, you will be stuck the first time a layout breaks on a phone, and it will break.
This is for people who can write basic HTML and CSS and keep getting layouts that break on phones. You will build responsive pages with Flexbox and Grid, reuse components, and fix the overlap and overflow bugs that show up after launch. If you keep stacking float hacks and random media queries, every new page will reopen the same fights.
This is for people who already build responsive pages and now need styles that survive real traffic, real content and a real team. You will structure CSS with layers and tokens, lay out with Grid and container queries, ship faster pages, and audit accessibility before launch. If you keep adding rules on top of rules, every release will be harder than the last.