Stacking DIVs on top of each other?

Is it possible to stack up multiple DIVs like:

<div>
<div></div>
<div></div>
<div></div>
<div></div>
</div>

So that all those inner DIVs have the same X and Y position? By default they all go below each other increasing the Y position by the height of the last previous DIV.

I have a feeling some sort of float or display or other trick could bite?

EDIT: The parent DIV has position relative, so, using position absolute does not seem to work.

Solution 1:

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>

Solution 2:

To add to Dave's answer:

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

Solution 3:

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;
}