What is the function of CSS in web design?

CSS, or Cascading Style Sheets, plays a crucial role in web design, specifically in controlling the layout and appearance of HTML elements. It allows designers to separate content from presentation, meaning that the style and visual design can be modified without altering the underlying HTML structure. With CSS, developers can define styles for various elements, such as fonts, colors, spacing, and positioning, providing a consistent and aesthetically pleasing look across web pages.

By using CSS, designers can create responsive web layouts that adapt to different screen sizes and devices, enhancing user experience. It also enables the implementation of visual effects and animations, further enriching the design without compromising on content structure.

The other options relate to different aspects of web design: while structuring content is primarily handled by HTML, aspects such as interactivity are more closely associated with JavaScript. Website performance optimization is mainly achieved through various techniques that include but aren't limited to resource management and compression, which fall outside the direct function of CSS. Thus, the unique capability of CSS to precisely dictate the visual presentation makes it an essential tool in web design.