What is Cascading Style Sheets (CSS) primarily used for?

Cascading Style Sheets (CSS) is primarily used to describe the presentation of a document written in HTML or XML (including dialects such as SVG or XHTML). It allows web developers to apply styles, such as layouts, colors, fonts, and spacing, to web pages. By separating content from design, CSS provides greater flexibility in how the content is displayed across different devices and screen sizes.

Using CSS, you can control multiple web pages at once by defining a single style sheet, which maintains consistency in design elements throughout a website. This separation also makes it easier to manage, maintain, and update a website's aesthetics without altering the underlying HTML structure. The effectiveness of CSS lies in its ability to enhance user experience by providing visual appeal and improving the functionality of web documents.

The other options do not align with the primary purpose of CSS, as graphic design for images focuses more on image creation and manipulation rather than styling web documents, audio quality enhancement involves different technologies like audio codecs, and file download facilitation is typically managed through server-side scripts and HTML links, rather than through CSS.