What does the z-index property control in CSS?

The z-index property in CSS is integral in managing how elements overlap within a webpage's layout. Specifically, it controls the stacking order of elements, allowing developers to specify which elements should appear on top of others when they overlap. By assigning a higher z-index value to an element, it will be rendered above elements with lower z-index values. This is particularly useful in creating layers of content, such as dropdown menus, modals, or image galleries.

Understanding the z-index property is crucial for web design because it influences the visual hierarchy of page elements. For example, if a user wants a modal dialog to appear above the main content of a webpage, they would assign a higher z-index to the modal than that of the content.

The other options do not accurately describe the specific function of the z-index property. While visibility, border size, and text arrangement are all important aspects of CSS, they are controlled by different properties and do not directly relate to the stacking order of elements.