Which tool is specifically designed for formatting and styling website user interfaces?

CSS, or Cascading Style Sheets, is the tool specifically designed for formatting and styling website user interfaces. It allows developers to control the layout, colors, fonts, and overall visual appearance of web pages. By separating content (HTML) from presentation (CSS), developers can create more maintainable and visually appealing websites.

CSS provides a range of properties that can be applied to HTML elements, enabling intricate designs and responsive layouts. This ability to style elements with precision is essential for enhancing user experience and aesthetic appeal. CSS is fundamental for building modern web interfaces, making it the correct choice for this question.