z-index is used in CSS to control the stacking order of elements, but its effect is limited by the "stacking context". 1. z-index only takes effect in the same stacking context, and the higher the value, the higher the value. 2. The stacking context is created by specific conditions, such as positioning elements to set z-index, transparency, transformation, filters, etc. 3. Children elements are always stacked in the parent context and cannot break through the parent hierarchy. 4. Avoid abuse of high values ??when using z-index, adopt meaningful hierarchy ranges, and check whether the parent element affects stacking. 5. When encountering problems, DOM structure and style should be reviewed to confirm the context relationship. The key to understanding z-index lies in mastering the mechanism of stacking context.

The z-index property in CSS controls the stacking order of elements on a webpage. It determines which elements appear in front of or behind others when they overlap. However, how it works isn't always straightforward because z-index doesn't operate in isolation—it's closely tied to what's known as the "stacking context."

What Does z-index Actually Do?

At its core, z-index sets the vertical stacking order of positioned elements (like those with position: relative , absolute , fixed , or sticky ). A higher z-index value means the element will appear above others with lower values.

But here's the catch: it only works within the same stacking context . You could give an element a z-index of 9999, but if it's inside a stacking context that's itself lower than another context, it'll still appear behind.

Think of stacking contexts like layers in graphic design software—each has its own internal hierarchy, and the top layer in one group can still be beneath the bottom layer of another group.

How Stacking Contexts Are Created

A stacking context is formed in a few specific ways:

• When an element has a position value other than static and also has a z-index value (other than auto )
• When an element has an opacity less than 1
• When using transforms ( translate , scale , etc.)
• With filter effects
• Using will-change or contain properties
• With perspective or clip-path

Once a stacking context is created, all of its child elements are stacked relative to it—not to the global document.

This is why setting a high z-index on a deeply nested element might not help if its parent has a lower stacking level. The child can't escape its parent's stacking context.

Practical Tips for Using z-index Effectively

Here are some things to keep in mind when working with z-index :

• Don't overuse high values ??like 99999 – It makes debugging harder and suggests you're fighting the structure rather than working with it.
• Use meaningful ranges – For example, headers at 100 , modals around 200 , tooltips at 300 — this keeps your stacking logic organized.
• Keep related elements in the same stacking context – This helps maintain predictable layering.
• Test visually – Sometimes overlapping isn't due to z-index at all—it could be layout flow or positioning issues.

If you're troubleshooting unexpected stacking behavior, inspect the DOM tree and look for any parent elements that may have inadvertently created a new stacking context.

Final Thoughts

Understanding z-index means understanding stacking contexts. One doesn't work without the other. While it seems simple at first glance, things get complex quickly once multiple contexts come into play.

So, remember:

• z-index only affects elements in the same stacking context
• New stacking contexts can be created by various CSS properties
• Always check parent elements when things aren't stacking as expected

And really, that's most of what you need to know.

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