What is the CSS z-index property and how does it work?
Aug 01, 2025 am 07:07 AMz-index controls the stacking order of overlapping positioned elements, with higher values appearing in front; 1. It only applies to positioned elements (not static); 2. Higher z-index values stack above lower or negative ones; 3. Elements with the same z-index stack per HTML order; 4. Stacking contexts limit z-index scope when created by properties like position with z-index, opacity
The z-index
property in CSS controls the vertical stacking order of elements that overlap. It determines which elements appear in front of or behind others on the page.

How z-index Works
By default, elements are stacked in the order they appear in the HTML — later elements appear on top of earlier ones. But when you want more control over this layering (like placing a dropdown menu above other content), z-index
comes in.
z-index
only works on positioned elements — that means elements with a position
value other than static
(so relative
, absolute
, fixed
, or sticky
).

Here’s how it works:
- Elements with a higher
z-index
value appear in front of elements with a lower one. - Elements with a lower or negative
z-index
appear behind. - If two elements have the same
z-index
, the one that comes later in the HTML source wins.
.box1 { position: absolute; z-index: 1; } .box2 { position: absolute; z-index: 3; /* This will appear in front of .box1 */ }
Key Points to Remember
Positive, zero, and negative values are allowed.
Evenz-index: -1
is valid — it places the element behind elements withz-index: 0
or the default stacking context.Stacking contexts matter.
z-index
doesn’t always behave globally. A new stacking context is created by elements with:position
z-index
(other thanauto
)opacity
less than 1transform
,filter
,will-change
, etc.
Once a stacking context is created, the
z-index
of child elements only applies within that context.Don’t overuse high values like 9999.
While common, it can lead to conflicts later. Use small, logical values (like 1, 2, 3) for components (modals, tooltips, headers) so your layering stays manageable.
Practical Example
<div class="box red"></div> <div class="box blue"></div>
.box { position: absolute; width: 100px; height: 100px; } .red { background: red; left: 50px; top: 50px; z-index: 2; } .blue { background: blue; left: 80px; top: 80px; z-index: 1; }
Even though .blue
comes after .red
in the HTML, .red
appears in front because it has a higher z-index
.
Basically, z-index
is like layers in a design tool — think Photoshop or Figma. You’re assigning which elements sit on top, but only if they’re positioned. And be mindful of stacking contexts — they can trap your high z-index
values inside a lower-level container.
It’s simple in concept, but tricky in complex layouts. Use it thoughtfully.
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