The first websites I built were using html only. They were functional, but not sophisticated. It wasn’t until I started using software which converted a page of text and graphics to html, and later, WordPress, that I was able to build websites that had anything beyond basic functionality.
CSS (and SASS) is what helps give a website structure and better functionality. The box model, in particular, defines the page. Additional CSS selectors can define characteristics such as line height, fonts, letter sizes, and so forth. These selectors can apply to just one line, to a section of the web page, or the entire web page. Other selectors can be used to structure menus and drop-down menus. The variety and versatility of choices means a wide variety of web formats can be built.
I went over 26 sections at atozcss.com in an afternoon. I would have found them even more helpful had I known about this site before starting code school, because then I could have taken the time to go over the material in more depth, and perhaps test out some sections of code. Showing the code alongside how a web page featuring the code appeared would have been even more useful, in that I could have seen the one-to-one correspondence between the code and the page appearance.
Nonetheless, I have noted the atozcss.com site for future reference, and may find myself referring to it.