Codecademy says that a markup language like HTML “defines structure and presentation of raw text”. Doesn’t CSS define the presentation of web content?
That is correct! These days HTML is best categorized as a descriptive markup language which encourages developers to focus on the semantic meaning of information rather than its visual appearance. However, there are other markup languages which make no such separation of concerns. In fact, HTML used to be more presentational before the advent of CSS.
On a related note, it is important to realize that most browsers have default styles which they will apply to certain HTML elements.