What best describes the function of CSS in web development?

CSS, or Cascading Style Sheets, primarily serves the function of styling and formatting web pages. It allows developers to define the look and layout of HTML elements, offering control over aspects such as colors, fonts, spacing, and positioning. By separating content (HTML) from design (CSS), developers can create visually appealing websites that are also easier to maintain.

Using CSS, web developers can apply consistent styling across multiple pages of a website, enhance user experience through responsive design, and implement various design techniques like flexbox and grid layouts. This separation of structure and presentation is fundamental to modern web development practices, making it easier to adapt designs and maintain code over time. In contrast, the other options pertain to functions that are not part of CSS's role in web development. For example, server-side application creation, data storage, and user authentication are handled by backend technologies and programming languages rather than by CSS.