HTML and CSS are not realted in any way

HTML and CSS really aren't similiar in any way - one was designed to compliment and make the other one look better. CSS simply puts an extra coat of paint on HTML, which natively has very few design choices. CSS is most definetly NOT a replacement for HTML. The language simply cannot dictate certain rendering things like a body or a header, or even the text on something like a button. HTML tells the computer what to render basically, and CSS finishes it by telling the web browser how to render it. They're about as separate as oil and water in that regard.