The acronym "CSS" stands for Cascading Style Sheets, and basically all that that wording means is that it's a document (sheet) that lays out the (styles) of each and every element in a webpage/app, and that those styles have a certain (cascading) order in which they're implemented. CSS is not a language that can really exist on its own. To put it in terms of the English language, it's like if HTML is a noun, then CSS is its adjective. If someone approaches a conversation that is already underway, they might ask, "What are you guys talking about?" and while an appropriate answer would include a noun and build upon that, simply replying with an adjective would sound rather silly. Picture it. "What are you guys talking about?" "Pretty." That makes no sense, right? That's kind-of why I say CSS is a supporting language. The conversation might continue: "Pretty ...what?" "Pretty flowers." Ah! Now we have a noun to which we can apply our adjective, and life makes a little more sense.