CSS Positioning: Understanding z index: The stacking context

Stacking context is the three-dimensional conceptualization of HTML elements along an imaginary z-axis relative to the user who is assumed to be facing the viewport or the webpage. HTML elements occupy this space in priority order based on element attributes.

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The stacking context

In the previous example, Adding z-index, the rendering order of certain DIVs is influenced by their z-index value. This occurs because these DIVs have special properties which cause them to form a stacking context.

A stacking context is formed, anywhere in the document, by any element which is either

• the root element (HTML),
• positioned (absolutely or relatively) with a z-index value other than "auto",
• a flex item with a z-index value other than "auto",that is the parent element display: flex|inline-flex,
• elements with an opacity value less than 1. (See the specification for opacity),
• elements with a transform value other than "none",
• elements with a mix-blend-mode value other than "normal",
• elements with a filter value other than "none",
• elements with a perspective value other than "none",
• elements with isolation set to "isolate",
• position: fixed
• specifying any attribute above in will-change even if you don't specify values for these attributes directly (See this post)
• elements with -webkit-overflow-scrolling set to "touch"

Within a stacking context, child elements are stacked according to the same rules previously explained. Importantly, the z-index values of its child stacking contexts only have meaning in this parent. Stacking contexts are treated atomically as a single unit in the parent stacking context.

In summary:

• Positioning and assigning a z-index value to an HTML element creates a stacking context, (as does assigning non-full opacity).
• Stacking contexts can be contained in other stacking contexts, and together create a hierarchy of stacking contexts.
• Each stacking context is completely independent from its siblings: only descendant elements are considered when stacking is processed.
• Each stacking context is self-contained: after the element's contents are stacked, the whole element is considered in the stacking order of the parent stacking context.

The example

In this example every positioned element creates its own stacking context, because of their positioning and z-index values. The hierarchy of stacking contexts is organized as follows:

• Root
  • DIV #1
  • DIV #2
  • DIV #3
    • DIV #4
    • DIV #5
    • DIV #6

It is important to note that DIV #4, DIV #5 and DIV #6 are children of DIV #3, so stacking of those elements is completely resolved within DIV#3. Once stacking and rendering within DIV #3 is completed, the whole DIV #3 element is passed for stacking in the root element with respect to its sibling's DIV.

Example

HTML

    <div id="div1">
      <h1>Division Element #1</h1>
      <code>position: relative;<br/>
      z-index: 5;</code>
    </div>

    <div id="div2">
      <h1>Division Element #2</h1>
      <code>position: relative;<br/>
      z-index: 2;</code>
    </div>

    <div id="div3">

      <div id="div4">
        <h1>Division Element #4</h1>
        <code>position: relative;<br/>
        z-index: 6;</code>
      </div>

      <h1>Division Element #3</h1>
      <code>position: absolute;<br/>
      z-index: 4;</code>

      <div id="div5">
        <h1>Division Element #5</h1>
        <code>position: relative;<br/>
        z-index: 1;</code>
      </div>
   
      <div id="div6">
        <h1>Division Element #6</h1>
        <code>position: absolute;<br/>
        z-index: 3;</code>
      </div>
    </div>

CSS

* {
    margin: 0;
}
html {
    padding: 20px;
    font: 12px/20px Arial, sans-serif;
}
div {
    opacity: 0.7;
    position: relative;
}
h1 {
    font: inherit;
    font-weight: bold;
}
#div1,
#div2 {
    border: 1px dashed #696;
    padding: 10px;
    background-color: #cfc;
}
#div1 {
    z-index: 5;
    margin-bottom: 190px;
}
#div2 {
    z-index: 2;
}
#div3 {
    z-index: 4;
    opacity: 1;
    position: absolute;
    top: 40px;
    left: 180px;
    width: 330px;
    border: 1px dashed #900;
    background-color: #fdd;
    padding: 40px 20px 20px;
}
#div4,
#div5 {
    border: 1px dashed #996;
    background-color: #ffc;
}
#div4 {
    z-index: 6;
    margin-bottom: 15px;
    padding: 25px 10px 5px;
}
#div5 {
    z-index: 1;
    margin-top: 15px;
    padding: 5px 10px;
}
#div6 {
    z-index: 3;
    position: absolute;
    top: 20px;
    left: 180px;
    width: 150px;
    height: 125px;
    border: 1px dashed #009;
    padding-top: 125px;
    background-color: #ddf;
    text-align: center;
}

Result

See also

• Stacking without z-index : Default stacking rules
• Stacking and float : How floating elements are handled
• Adding z-index : Using z-index to change default stacking
• Stacking context example 1 : 2-level HTML hierarchy, z-index on the last level
• Stacking context example 2 : 2-level HTML hierarchy, z-index on all levels
• Stacking context example 3 : 3-level HTML hierarchy, z-index on the second level