What are CSS and its purpose?

Cascading Style Sheets, commonly known as CSS, serve the primary purpose of controlling the presentation and layout of HTML elements on a web page. This allows web developers to separate content (written in HTML) from design aspects, such as colors, fonts, spacing, and positioning of elements. By using CSS, developers can create visually engaging and responsive designs that can easily adapt across different devices and screen sizes.

The "cascading" aspect of CSS refers to the way styles are applied in a hierarchy—styles can be defined in multiple places, and the browser resolves which styles to apply based on specificity and the order in which the styles are declared. This is critical for maintaining consistency and efficiency across a website.

In contrast, the other options do not accurately represent the purpose or definition of CSS, which contributes to the clarity and effectiveness of web design.