How does the z-index property really work?
Both negative and positive integers are allowed.
The position must be set on the element.
Before I get into those details, though, let me explain z-index from the ground up.
Every webpage is made up of what are called stacking contexts. You can think of these as, quite literally, a stack of elements. The z-index property determines the order of items in each stack, with higher z-index being placed further up.
All pages begin with a root stacking context, which builds from the root element (as you'd expect). But more stacking contexts can be created in a number of ways. One way is an absolutely positioned div; its children will be in a new stacking context.
The specs lists all of the instances that create a new stacking context. As others have stated, this includes explicitly positioned elements and will soon include elements that aren't completely opaque.
As I said before, z-index only takes effect if you explicitly set the position of the element. This means setting it to be fixed, absolute, or relative. This is best shown through example, I think.
In this example, we'd expect the blue div to be on top of the grey one given its z-index, right? But, as you can see, it's on the bottom. This is, of course, because we haven't set its position. Once we do that it displays as we'd expect. Again, you must set the position.
The specs also tell us that negative values are fine. With that said, you don't need to use negative values. It's perfectly fine to use positive integers, too. The default z-index value for an element is 0.
For the record, w3schools is a notoriously unreliable source for learning. While it can be a quick and convenient resource, there are lots of gaps in their information, and at times even wrong information. I recommend you to use more reliable sources like the Mozilla Developer Network, and also the specs themselves.
Before I start explaining, let me note that z-index only has an affect if the element has rendered value of position:relative, position:absolute, or position:fixed (NOT static) because each of these make it have its own stacking context. That means that values like initial or inherit may or may not work either depending on the situation.
Also note that in this post I'll be using the format 1.1.1 to signify that I am selecting the first element's first child's first child. 2.1.1 would be the second element's first child's first child and so on.
I think z-index is best explained with an analogy using sub lists. Let's start with the simplest example:
<div class="top-level"></div>
<div class="top-level sibling"></div>
We can represent this in terms of a list like so:
- Top level
- Top level sibling
Now by default the ones further down the list will render on top of the ones before it. So in this case 2 will be positioned on top of 1.
What z-index allows us to do is essentially reorder this list (within some bounds). The higher the z-index, the further down the list our element is.
I'll use inline CSS here to make showing it easy but you should definitely avoid inline CSS in production code.
<div class="top-level" style="z-index: 1;"></div>
<div class="top-level sibling"></div>
This now changes our sub listing to look like this:
- Top level sibling
- Top level - z-index:1
Great! Now the first element in our HTML will render on top of our second.
Where this gets more tricky is when we're dealing with children (nested) elements.
An easier way to think about this situation is to think that when an element starts being rendered it will render all children of the element before moving onto any siblings.
Also keep in mind that sub-lists cannot change levels, meaning they cannot be on the same level of their parent or their children elements.
That means that if we have the following:
<div class="top-level">
<div class="sub-level" style="z-index: 1;"></div>
</div>
<div class="top-level sibling"></div>
Our rendering sub lists will look like the following:
- Top level
- Sub level - z-index1
- Top level sibling
Thus, we look our top level and see which one is at the bottom of the list. In this case, 2.0 is, so it will be on top of 1.0. Then we look and see if it has any sub lists (children). It doesn't, so we go to 1.0.
1.0 has a child, 1.1, which will be visually above 1.0 (just like it would be if we didn't give it a z-index), but it will still be below 2.0 because 1.0 is below 2.0.
Thus the z-index here doesn't help us out because 1.1 doesn't have any siblings.
Let's take a slightly more complex example:
<div class="top-level">
<div class="sub-level" style="z-index: 2;"></div>
<div class="sub-level sibling"></div>
<div class="sub-level sibling" style="z-index: 1;"></div>
</div>
<div class="top-level sibling">
<div class="sub-level"></div>
</div>
What's the sub listing for this example?
I'll give it to you, but it's good to try and do by yourself.
- Top level
- Sub level sibling
- Sub level sibling - z-index1
- Sub level - z-index2
- Top level sibling
- Sub level
Thus, in terms of what the order is visually from top to bottom, the order goes 2.1, 2.0, 1.3, 1.2. 1.1, 1.0.
That's not so bad, is it?
This behavior is true no matter how far deep or how many siblings are there.
The only exception to the rule that children are rendered above parents is when the children have a negative z-index. When a negative z-index is given, it places itself below a parent element.
Thus if we have the following:
<div class="top-level">
<div class="sub-level" style="z-index: -1;"></div>
</div>
<div class="top-level sibling"></div>
The sub list tree would look like this:
- Sub level - z-index-1
- Top level
- Top level sibling
And the top to bottom layering would be 2.0, 1.0, 1.1.
A slightly more complex example:
<div class="top-level">
<div class="sub-level" style="z-index: -1;"></div>
<div class="sub-level sibling"></div>
</div>
<div class="top-level sibling"></div>
List representation:
- Sub level - z-index-1
- Top level
- Sub level
- Top level
- Top level sibling
But you should avoid negative z-indexes. If you think you need them it is likely that your HTML is structured improperly.
That's about it! If you're still interested in learning more, reading the specs is always good.
Keep in mind that other properties, including but not limited to opacity, transform, and will-change, create their own stacking context and may have an affect on the rendering order of elements.
opacity works similarly to z-index - a child can only be as opaque as its parent - but it can't have negative values.
The only exception to the rule that children are rendered above parents is when the children have a negative z-index. When a negative z-index is given, it places itself below a parent element.