Z-Index Behaviour on Pseudo Elements

Using a ::before pseudo element on UL to create a time line representation

You can use a white background, padding and relative positioning to hide part of the timeline with each item:

.container ul{  position: relative;}.container ul::before{  content: "";  position: absolute;  top: 0px;  bottom: 0px;  border: 1px dashed red;}.container ul li {  display: block;  position: relative;  left: -20px;  background-color: white;  padding: 5px;  padding-left: 20px;  margin: 40px 0px;}

<div class="container">  <ul>    <li>First</li>    <li>Second</li>    <li>Third</li>    <li>Fourth</li>  </ul></div>

Unexpected behaviour: pseudo elements and display: table-cell

The ::after pseudo-element is rendered as a child of your div, so instead of populating the same row as the div cell, it lives in its own anonymous block-level table within the div cell, on its own line, beneath the text "one".

If your div is displayed as a table row instead, then the text "one" will be contained within its own anonymous table cell, and the ::after pseudo-element in another cell of its own on the same row:

div {  display: table-row;}
div::after {  content: "two";  display: table-cell;}

<div>one</div>

Why can't an element with a z-index value cover its child?

There are two important things you need to know: the painting order and the stacking context. If you refer to the specification, you can find how and when elements are painted.

3. Stacking contexts formed by positioned descendants with negative z-indices (excluding 0) in z-index order (most negative first) then tree order.

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8. All positioned, opacity or transform descendants, in tree order that fall into the following categories:

   1. All positioned descendants with 'z-index: auto' or 'z-index: 0', in tree order.

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9. Stacking contexts formed by positioned descendants with z-indices greater than or equal to 1 in z-index order (smallest first) then tree order.

It's clear from this that we first paint elements with negative z-index at step (3), then the one with z-index equal to 0 at step (8), and finally the ones with positive z-index at step (9), which is logical. We can also read in another part of the specification:

Each box belongs to one stacking context. Each box in a given stacking context has an integer stack level, which is its position on the z-axis relative to other boxes in the same stacking context. Boxes with greater stack levels are always formatted in front of boxes with lower stack levels. Boxes may have negative stack levels. Boxes with the same stack level in a stacking context are stacked bottom-to-top according to document tree order.

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To understand when each element will be painted you need to know its stacking context and its stack level inside this stacking context (defined by z-index). You also need to know whether that element establishes a stacking context. This is the tricky part, because setting z-index will do this:

For a positioned box, the z-index property specifies:

1. The stack level of the box in the current stacking context.
2. Whether the box establishes a stacking context

Values have the following meanings:

<integer>

This integer is the stack level of the generated box in the current stacking context. The box also establishes a new stacking context.

auto

The stack level of the generated box in the current stacking context is 0. The box does not establish a new stacking context unless it is the root element.

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Now we have all the information to better understand each case. If the parent element has a z-index value of something other than auto, then it will create a stacking context, thus the child element will be painted inside whatever their z-index is (negative or positive). The z-index of the child element will simply tell us the order of painting inside the parent element (this covers your second point).

Now, if only the child element has a positive z-index and we set nothing on the parent element, then considering the painting order, the child will be painted later (in step (9)) and the parent in step (8). The only logical way to paint the parent above is to increase the z-index, but doing this will make us fall into the previous case where the parent will establish a stacking context and the child element will belong to it.

There is no way to have the parent above a child element when setting a positive z-index to the child. Also there is no way to have the parent above the child if we set a z-index to the parent element different from auto (either positive or negative).¹

The only case where we can have a child below its parent is to set a negative z-index on the child element and keep the parent at z-index: auto, thus this one will not create a stacking context and following the painting order the child will be painted first.

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In addition to z-index, there are other properties that create a stacking context. In case you face an expected stacking order, you need to consider those properties, too, in order to see if there is a stacking context created.

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Some important facts that we can conclude from the above:

1. Stacking contexts can be contained in other stacking contexts, and together create a hierarchy of stacking contexts.
2. Each stacking context is completely independent of its siblings: only descendant elements are considered when stacking is processed.
3. Each stacking context is self-contained: after the element's contents are stacked, the whole element is considered in the stacking order of the parent stacking context. ^ref

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¹: there is some hacky ways if we consider the use of 3D transformation.

Example with an element going under its parent element even if this one has a z-index specified.

.box {
  position:relative;
  z-index:0;
  height:80px;
  background:blue;
  transform-style: preserve-3d; /* This is important */
}
.box > div {
  margin:0 50px;
  height:100px;
  background:red; 
  z-index:-1; /* this will do nothing */
  transform:translateZ(-1px); /* this will do the magic */
}

<div class="box">
  <div></div>
</div>

Strange CSS pseudo / Chrome behaviour

It appears to be a redraw issue within Chrome. A heavy animation (fa-spin) from FontAwesome killed the redrawing for the pseudo element.

To get the right redrawing power back to the ::before element I used translate3D trick to get the GPU up to speed for this element.

&::before {
  -webkit-transform: translate3d(0, 0, 0);
}

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