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Table of Contents
How Stacking Context Works
When Is a Stacking Context Created?
Practical Example
Why It Matters
Home Web Front-end CSS Tutorial What is a stacking context?

What is a stacking context?

Jul 27, 2025 am 03:55 AM
css Stacking context

A stacking context is a self-contained layer in CSS that controls the z-order of overlapping elements, where nested contexts restrict z-index interactions; it is created by properties like z-index on positioned elements, opacity

What is a stacking context?

A stacking context is a three-dimensional conceptual layer in CSS that determines the visual order of elements on a webpage when they overlap. Even though web pages are built in a 2D plane, CSS allows elements to stack on top of one another — like sheets of paper piled up — and the stacking context defines which elements appear above or below others.

What is a stacking context?

Think of it like this: when elements overlap (due to positioning, opacity, flexbox, etc.), the browser needs rules to decide who shows up on top. That’s where the stacking context comes in.

How Stacking Context Works

Elements don’t all compete globally for z-order. Instead, stacking contexts can be nested. Each stacking context is self-contained, meaning elements inside it don’t affect elements in other stacking contexts. The browser paints elements in a specific order, from back to front:

What is a stacking context?
  1. Background and borders of the parent element
  2. Block-level elements in normal flow, in order of appearance
  3. Positioned elements (and flex/grid children), in order of their z-index:
    • First, those with negative z-index (painted back to front)
    • Then, non-positioned elements and those with z-index: auto
    • Finally, elements with positive z-index (painted front to back)

But here’s the key: a new stacking context limits how z-index values interact.

When Is a Stacking Context Created?

A new stacking context is formed when any of the following are applied to an element:

What is a stacking context?
  • position with z-index other than auto (only if the element is positioned: relative, absolute, fixed, or sticky)
  • opacity less than 1
  • transform (e.g., scale, rotate)
  • filter (e.g., blur(2px))
  • will-change (if it hints at transform or opacity)
  • perspective or clip-path
  • isolation: isolate
  • mix-blend-mode other than normal
  • contain with value layout, paint, or strict

Once a new stacking context is created, any z-index values inside are relative to that context, not the whole page.

Practical Example

<div class="box1">Box 1 (z-index: 2)</div>
<div class="container">
  <div class="box2">Box 2 (z-index: 100)</div>
</div>
.box1 {
  position: relative;
  z-index: 2;
  background: red;
}

.container {
  position: relative;
  z-index: 1; /* Creates a new stacking context */
  opacity: 0.9; /* Also creates a new stacking context */
}

.box2 {
  position: relative;
  z-index: 100;
  background: blue;
}

Even though .box2 has z-index: 100, it’s contained within the stacking context of .container, which has a lower effective stacking level than .box1. So red .box1 will appear above blue .box2 — counterintuitive, but correct.

Why It Matters

Understanding stacking contexts helps avoid frustrating layout bugs. For example:

  • A dropdown menu getting "cut off" by a nearby element
  • A modal not appearing on top despite high z-index
  • Tooltips or popovers appearing behind other content

The fix often involves checking if a parent element created an unintended stacking context (e.g., via opacity or transform), which then traps the child’s z-index.

Basically, it’s not just about z-index — it’s about the hierarchy of stacking contexts.

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