What does the z-index property control in CSS?

The z-index property in CSS is crucial for controlling the stacking order of elements that overlap on a web page. When multiple elements are displayed in the same 3D space, the z-index determines which elements appear in front of or behind others. This is particularly useful in complex layouts where elements, like images, divs, or other containers, are positioned with CSS properties such as position: relative, position: absolute, or position: fixed.

With regard to this property, only elements that have a defined positioning context (relative, absolute, or fixed) will respond to z-index values. A higher z-index value will place an element closer to the front of the stacking context, while a lower value will push it to the back. This capability allows for more control over visual hierarchy and layering of web content, which is essential in web design.

The other choices address different aspects of CSS but do not relate to the z-index property. Visibility is managed with properties like visibility and display; font styling is controlled using font-related properties such as font-family and font-size; and container width is adjusted with width and max-width properties. Thus, the focus on stacking order accurately captures the functionality and importance of the z-index property in web design.