Is Z-Index Absolute or Relative?

Z-Index: Relative or Absolute?

When dealing with the z-index property, it's essential to understand its nature as either an absolute or relative stack order. The answer lies in its relative nature, as demonstrated by the 'z-index model' outlined below.

Z-Index is Relative

The z-index property determines the stacking order of elements in a web page. However, its position is not absolute within the page but rather relative to its parent element. This means that an element with a higher z-index will appear in front of its siblings within the same parent.

Z-Index Determination

The mechanism for determining the z-index follows a hierarchical process:

1. Initial Z-Index Assignment: Direct child nodes of the body element are assigned an initial z-index value of 1 in ascending order.
2. Manual Z-Index Adjustment: If an element has a manually set z-index property, its value is considered.
3. Position in Document Tree: In case of equal z-index values, the elements' relative position in the document tree determines their stacking order.

Example

Consider the following HTML code:

<div id="X" style="z-index:1">
  <div id="Y" style="z-index:3"></div>
</div>
<div id="Z" style="z-index:2"></div>

While it may seem intuitive that #Y would overlap #Z due to its higher z-index, the reality is different. As #Y is a direct child of #X (with a z-index of 1), #Z will appear in front of both #X and #Y due to its higher stacking order within the document tree.

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