Which of the following is NOT a function of CSS in web design?

The function of CSS (Cascading Style Sheets) primarily focuses on the presentation and layout of web pages, rather than the structural elements themselves. While CSS can indeed manipulate how elements are displayed, including styling text and layouts, creating visual effects such as gradients, and establishing responsive design rules to ensure that web pages look good on various devices, it does not fundamentally define the website's structure.

Structure is typically established through HTML (Hypertext Markup Language), which dictates the layout and hierarchy of the content, such as headings, paragraphs, lists, and other semantic elements. CSS works on top of this structure to enhance its visual appearance. Therefore, defining the structure of a website falls outside the responsibilities of CSS, making this the correct answer in distinguishing the functions of CSS from those of HTML.