I have a code like this (divs have absolute positions and overlap each other at least partially):
<div style="z-index: 1">
<div style="z-index: 3">
</div>
</div>
<div style="z-index: 2">
<div style="z-index: 4">
</div>
</div>
It seems to work okay in the browsers I need, but somehow it doesn't feel right. So my question is: should I consider another approach or this technique is completely legit?
In CSS, z-index values are relative to stacking contexts, which do somewhat rely on hierarchy.
As all your divs are absolutely positioned and have a z-index that is not auto, every one of them establishes a stacking context for their descendants. So the first inner div participates in the first outer div's stacking context, and its z-index is relative to that stacking context. Likewise, the second inner div participates in the second outer div's stacking context, and its z-index is relative to that stacking context.
Because they are confined to their own parent stacking contexts, neither of the two inner divs is aware of the other. In other words, whatever z-index you set to one inner div will never have an effect on the other.
However, assuming everything else is defaulted, the two outer divs participate in the same parent stacking context, so the second can be said to have a higher z-index (or higher stack level) than the first, and swapping their z-index values will indeed cause the one with the higher stack level to then appear in front of the one with the lower stack level.
All of this is defined clearly in section 9.9 of CSS2.1, and can be relied on, bugs notwithstanding.
I was trying to answer this question: How to make a colorful gradient glow around your input-box?
My problem and question is why does the rainbowBg div appear over the input and not behind it? I have tried positioning the input absolute, setting z-indexes, nothing worked.
<div class="rainbowWrap">
<div class="rainbowBg"></div>
<input class="rainbow" type="text"/>
</div>
Try typing in the input. You can only focus it if you click between the input border and the rainbowBg div. I have only tested in Chrome.
http://jsfiddle.net/b03acbdu/4/
z-index only works when you define position. Add "position: relative" to your rainbow class.
Just give the div a negative z-index. You can even remove z-index from the input then.
.rainbowBg {
z-index: -1;
}
http://jsfiddle.net/b03acbdu/6/
It goes behind the input because z-index is ingored for non-positioned elements and so givern a fixed value of 0. From MDN:
When no z-index property is specified, elements are rendered on the default rendering layer 0 (zero).
In terms of z-index being relative to parent or the whole document, MDN again has a useful article describing The Stacking Context. The children of .rainbowWrap are put into their own stacking context, so because .rainbowBg is a child of it, it will always be "on top of" it's prent's background. Or as the article puts it better:
An easy way to figure out the rendering order of stacked elements along the Z axis is to think of it as a "version number" of sorts, where child elements are minor version numbers underneath their parent's major version numbers.
I am having a z-index issue. I have a couple of div that sit on top of each other, within them div there is some content which shows on mouseover. This content is currently being displayed behind the parent div.
The parent div have a z-index: 2 as they need to be displayed above there own parent.
<div class="activity-display"><!--parent div z-index: 2-->
Running<!--This is displayed on hover underneath its parent div, but gets hidden underneath the below activity-display-->
</div>
<div class="activity-display">
Running
</div>
I have tried adding a higher z-index to the anchor and this doesn't solve the issue
Any help would be greatly appreciated
When using the z-index property elements must be positioned. Change the a tag to position:absolute.
You should at least give the elements you want to work with z-index a relative position, because z-index doesn't works on a static position. So really any other than the default position works.
You also don't need to rearrange children or parents elements, because children elements will always be over the parent. In other words, a child element will always have a z-index + 1 relative to the parent.
Please make a fiddle, so that we can help.
Also, some points to keep in mind, z-index works for elements position absolute.
How does z-index actually work?
Does it function on elements with no specified position?
Does it favor elements (i.e. make them on top) that have a specified position?
Do the numbers have to be negative like this?
<div style='z-index:-2;'>below</div>
<div style='z-index:-1;'>less below</div>
<div style='z-index:0;'>on top</div>
Or not? Would all positive numbers (in increasing value) end up with the last being on the top, the middle in the middle, and the first on the bottom?
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 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.
The css property z-index only works on positioned elements, meaning elements must be position absolute, fixed or relative in order for the z-index property to take effect.
The higher the z-index the closer to the front it will appear. The values specified for the z-index property can be positive or negative. A positioned element with a z-index value of 4 will appear above a positioned element with a z-index value of 3.
In CSS 2.1, each box has a position in three dimensions. In addition
to their horizontal and vertical positions, boxes lie along a "z-axis"
and are formatted one on top of the other. Z-axis positions are
particularly relevant when boxes overlap visually.
This article on z-index is just the link you are looking for! He really explains it well.
Basically, the higher the number, the higher up on the stack the element will be. So 1 is on top of 0 and -1 is under the z-index of 0, however they should be kept positive, as there is no reason to use negatives; it's considered bad practice. The browser interprets it when it renders the page, like any other CSS or HTML code.
Fast question; if you have 2 divs, one absolutley positioned and one relative, it isnt doing the z-index, i have attached a jsfiddle.
If i make them both relative they go with the flow of the DOM and one goes above the other on the y axis rather than z, any ideas?
Example JSfiddle (thanks Onheiron for fixing the render in jsfiddle)
Screenshot of whats happening: http://i.imgur.com/Lo37y.png , the gray thing should be behind the image.
Thanks.
You can't make a child appear behind it's parent! You can only do that with siblings.
See your updated fiddle
To answer this question, I need to explain stacking context.
When you add a z-index to an element, it creates a stacking context. All child elements are then compared to this parent stacking context.
Your element "slideImg" has a z-index of 1000. Any child elements are then compared within the stacking context of this parent element. So the z-index of "shadow", which is 0, is actually "1000 + 0", because it is within the stacking context of "slideImg".
To put "shadow" behind "slideImg" you can reverse their nesting:
<div id="shadow">
<div id="slideImg"></div>
</div>
Or put them adjacent to each other, so they are each considered in the same stacking context.
<div id="shadow"></div>
<div id="slideImg"></div>
There is an explanation of z-index and Stacking Context here.