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HomeGlossary TermsZ-Index


In the context of web development and graphic design, the term "Z-index" refers to a CSS property that controls the stacking order of positioned elements on a web page along the z-axis. The z-axis is perpendicular to both the x-axis (horizontal) and y-axis (vertical) and determines the depth or layering of elements in the visual hierarchy.

Key points related to the Z-index property include:

  • Layering Elements: The Z-index property allows web developers and designers to control how elements are layered on top of each other within the three-dimensional space of a web page. Elements with a higher Z-index value appear visually on top of elements with lower values.
  • Default Stacking Order: Elements on a web page have a default stacking order based on their position in the HTML document. Elements that appear later in the document order are rendered on top of elements that appear earlier. The Z-index property provides a way to override this default order.
  • Integer Values: Z-index values are integers, and the higher the integer value, the higher the element is stacked in the visual hierarchy. Negative values are also valid and can be used to place elements behind the default stacking order.
  • Positioned Elements: The Z-index property applies to positioned elements, which are elements with a CSS position value other than "static" (e.g., "relative," "absolute," or "fixed").
  • Siblings and Children: The Z-index property is primarily effective when applied to sibling or child elements that overlap. It does not affect the stacking order of elements that are not in the same stacking context.
  • Stacking Contexts: Certain properties, such as "position: fixed" and "position: relative" with a Z-index value other than "auto," create stacking contexts. Understanding stacking contexts is crucial for predicting the visual outcome of Z-index values.
  • Global Stacking Context: The root element of an HTML document creates a global stacking context. Understanding the concept of stacking contexts is essential for managing the layering of elements effectively.
  • Inheritance: Z-index values are inherited by child elements, but the stacking context is reset for each positioned element. This means that a child element's Z-index is only compared to its siblings and not to the parent's siblings.
  • Use Cases: Z-index is commonly used to control the layering of elements in scenarios such as creating dropdown menus, modal overlays, and implementing complex layouts with overlapping elements.
  • Rendering Performance: Excessive use of high Z-index values or creating deep nesting of stacking contexts can impact rendering performance, so it's essential to use Z-index judiciously.

The Z-index property is a valuable tool for designers and developers to manage the visual hierarchy of elements in a web page, ensuring that the layout appears as intended and that elements are appropriately layered for a cohesive user experience.

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