[html] Why does z-index not work?

So if I understand z-index correctly, it would be perfect in this situation:

I want to place the bottom image (the tag/card) below the div above it. So you can't see the sharp edges. How do I do this?

z-index:-1 // on the image tag/card

or

z-index:100 // on the div above

doesn't work either. Neither does a combination of anything like this. How come?

In many cases an element must be positioned for z-index to work.

Indeed, applying position: relative to the elements in the question would likely solve the problem (but there's not enough code provided to know for sure).

Actually, position: fixed, position: absolute and position: sticky will also enable z-index, but those values also change the layout. With position: relative the layout isn't disturbed.

Essentially, as long as the element isn't position: static (the default setting) it is considered positioned and z-index will work.

Many answers to "Why isn't z-index working?" questions assert that z-index only works on positioned elements. As of CSS3, this is no longer true.

Elements that are flex items or grid items can use z-index even when position is static.

From the specs:

4.3. Flex Item Z-Ordering

Flex items paint exactly the same as inline blocks, except that order-modified document order is used in place of raw document order, and z-index values other than auto create a stacking context even if position is static.

5.4. Z-axis Ordering: the z-index property

The painting order of grid items is exactly the same as inline blocks, except that order-modified document order is used in place of raw document order, and z-index values other than auto create a stacking context even if position is static.

Here's a demonstration of z-index working on non-positioned flex items: https://jsfiddle.net/m0wddwxs/

If you set position to other value than static but your element's z-index still doesn't seem to work, it may be that some parent element has z-index set.

The stacking contexts have hierarchy, and each stacking context is considered in the stacking order of the parent's stacking context.

So with following html

div { border: 2px solid #000; width: 100px; height: 30px; margin: 10px; position: relative; background-color: #FFF; }_x000D_
#el3 { background-color: #F0F; width: 100px; height: 60px; top: -50px; }

<div id="el1" style="z-index: 5"></div>_x000D_
<div id="el2" style="z-index: 3">_x000D_
  <div id="el3" style="z-index: 8"></div>_x000D_
</div>

no matter how big the z-index of el3 will be set, it will always be under el1 because it's parent has lower stacking context. You can imagine stacking order as levels where stacking order of el3 is actually 3.8 which is lower than 5.

If you want to check stacking contexts of parent elements, you can use this:

var el = document.getElementById("#yourElement"); // or use $0 in chrome;
do {
    var styles = window.getComputedStyle(el);
    console.log(styles.zIndex, el);
} while(el.parentElement && (el = el.parentElement));

There is a great article about stacking contexts on MDN

Your elements need to have a position attribute. (e.g. absolute, relative, fixed) or z-index won't work.

Make sure that this element you would like to control with z-index does not have a parent with z-index property, because element is in a lower stacking context due to its parent’s z-index level.

Here's an example:

<section class="content">            
    <div class="modal"></div>
</section>

<div class="side-tab"></div>

// CSS //
.content {
    position: relative;
    z-index: 1;
}

.modal {
    position: fixed;
    z-index: 100;
}

.side-tab {
    position: fixed;
    z-index: 5;
}

In the example above, the modal has a higher z-index than the content, although the content will appear on top of the modal because "content" is the parent with a z-index property.

Here's an article that explains 4 reasons why z-index might not work: https://coder-coder.com/z-index-isnt-working/