Describe z-index and how a stacking context is formed

z-index is a CSS property that controls the stacking order of positioned elements on the z-axis, determining which elements appear in front of or behind others in the vertical stacking order.

The z-index property accepts integer values, where higher values place an element closer to the top of the stacking order. Elements with a higher z-index value will appear in front of elements with a lower value.

When multiple elements overlap in a web page, the z-index property allows you to control their visibility and layering. However, z-index only applies to elements that have been explicitly positioned (position property set to a value other than static, which is the default).

Stacking Context: A stacking context is formed when an element has a z-index value other than auto, and either its position property is set to a value other than static, or it has a CSS property that establishes a new stacking context (e.g., opacity less than 1, transform, filter, etc.).

The stacking context organizes elements into separate layers, where elements within the same stacking context are stacked relative to each other, independent of elements in other stacking contexts.

Some key points about stacking contexts:

  1. Stacking Order: The stacking order within a stacking context is determined by the z-index values of its child elements. Elements with a higher z-index value will appear in front of elements with a lower value within the same stacking context.
  2. Stacking Context Hierarchy: Stacking contexts can be nested within each other, creating a hierarchy of stacking contexts. Elements within a higher-level stacking context will appear in front of elements in lower-level stacking contexts, regardless of z-index values.
  3. Stacking Context Isolation: Elements within a stacking context are isolated from elements in other stacking contexts. This means that a high z-index value within a stacking context does not automatically place an element in front of elements in a different stacking context with a lower z-index value.
  4. Stacking Context Inheritance: In most cases, the stacking context is inherited by the child elements within it. This means that child elements will respect the stacking order established by the parent stacking context. However, there are exceptions where certain properties, such as opacity, can create a new stacking context and reset the stacking order.

Understanding stacking contexts and how elements are stacked within them is crucial for controlling the layering and visibility of elements in complex layouts with overlapping elements. It allows for precise control over which elements appear in front or behind others on the z-axis.

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