Stacking DIVs on top of each other?

Question

Position the outer div however you want, then position the inner divs using absolute. They'll all stack up.

.inner {
  position: absolute;
}

<div class="outer">
   <div class="inner">1</div>
   <div class="inner">2</div>
   <div class="inner">3</div>
   <div class="inner">4</div>
</div>

To add to Dave's answer:

div { position: relative; }
div div { position: absolute; top: 0; left: 0; }

You can now use CSS Grid to fix this.

<div class="outer">
  <div class="top"> </div>
  <div class="below"> </div>
</div>

And the css for this:

.outer {
  display: grid;
  grid-template: 1fr / 1fr;
  place-items: center;
}
.outer > * {
  grid-column: 1 / 1;
  grid-row: 1 / 1;
}
.outer .below {
  z-index: 2;
}
.outer .top {
  z-index: 1;
}

If you mean by literally putting one on the top of the other, one on the top (Same X, Y positions, but different Z position), try using the z-index CSS attribute. This should work (untested)

<div>
    <div style='z-index: 1'>1</div>
    <div style='z-index: 2'>2</div>
    <div style='z-index: 3'>3</div>
    <div style='z-index: 4'>4</div>
</div>

This should show 4 on the top of 3, 3 on the top of 2, and so on. The higher the z-index is, the higher the element is positioned on the z-axis. I hope this helped you :)

All the answers seem pretty old :) I'd prefer CSS grid for a better page layout (absolute divs can be overridden by other divs in the page.)

<div class="container">
  <div class="inner" style="background-color: white;"></div>
  <div class="inner" style="background-color: red;"></div>
  <div class="inner" style="background-color: green;"></div>
  <div class="inner" style="background-color: blue;"></div>
  <div class="inner" style="background-color: purple;"></div>
</div>

<style>
.container {
  width: 300px;
  height: 300px;
  margin: 0 auto;
  background-color: yellow;
  /* important part */
  display: grid;
  place-items: center;
  grid-template-areas:
                  "inner-div";
}

.inner {
  height: 100px;
  width: 100px;
  /* important part */
  grid-area: inner-div;
}
</style>

If you hide the purple div with CSS, you'll see the blue div is at the top.

Here's a working link

style="position:absolute"