HTML and CSS Certification 2025 – 400 Free Practice Questions to Pass the Exam

The z-index property in CSS serves to specify the stack order of elements that overlap one another. When elements are positioned using CSS (for example, with position values like relative, absolute, or fixed), they can overlap in the layout. The z-index allows developers to control which element appears on top of others when they occupy the same space on the screen.

Elements with a higher z-index value will be displayed above those with a lower z-index value, thus affecting their visual stacking. This is particularly useful in scenarios where you have multiple overlapping elements, such as dropdown menus, modals, or any other layered interface components. It is important to note that z-index only works on positioned elements; if an element is not positioned, its z-index will not have any effect.

The other choices pertain to different CSS functionalities: defining color refers to properties like color or background-color; controlling visibility relates to properties such as visibility or display; and setting margins involves the margin property, which controls the spacing around elements. Each of these serves distinct purposes and does not relate to the stacking context that z-index addresses.