What is the function of z-index in web page design?

The z-index is a crucial property in web design that determines the stacking order of elements on a webpage. It allows designers to control how elements overlap and interact visually. When elements are positioned, usually through CSS using properties like position: absolute, position: relative, or position: fixed, the z-index assigns a numerical value that dictates their vertical stacking order. Higher z-index values will place an element in front of those with lower values, affecting how content is presented to users. This is especially useful for creating layered effects, such as dropdown menus, modals, or overlapping images, ensuring that the most relevant or interactive elements are displayed prominently.

The other options do not accurately describe the function of z-index; for instance, it does not determine advertisement sizes, loading speeds, or types of content. Each of these functions relates to other specific properties or elements of web design rather than the z-index itself.