How Do You Stack Two Children Perfectly on Top of Each Other?

The Question

You have a parent container with two children, and you want both children to completely overlap — same position, same size, stacked like cards in a deck:

<div class="container">
  <div class="foo"></div>
  <div class="bar"></div>
</div>

So we need to pull .foo and .bar out of the normal document flow so they both sit in the exact same spot inside .container.

Solution 1: Absolute Positioning

The classic answer. Make the parent a positioning context, then absolutely position the children to fill it.

.container {
  width: 100px;
  height: 100px;
  position: relative; /* establishes the positioning context */
  background-color: #eee;
}

.foo, .bar {
  position: absolute; /* removes them from the flow */
  inset: 0;           /* shorthand for top/right/bottom/left: 0 */
}

.foo {
  background-color: rgba(255, 0, 0, 0.5);
  z-index: 1;
}

.bar {
  background-color: rgba(0, 0, 255, 0.5);
  z-index: 2; /* sits above .foo */
}

Why this works

• position: relative on the parent — without it, the children would anchor to the nearest positioned ancestor (or the viewport). This line locks them to .container.
• inset: 0 — a modern shorthand that stretches the child to fill the parent. No need to hard-code width and height.
• z-index — when elements overlap, the later one wins by default. Use z-index when you want explicit control over stacking order.

Solution 2: CSS Grid

If you'd rather not reach for position: absolute, CSS Grid has an elegant trick: put every child into the same grid cell.

.container {
  display: grid;
  width: 100px;
  height: 100px;
}

.foo, .bar {
  /* everyone claims row 1, column 1 */
  grid-area: 1 / 1;
}

That's it. Both children occupy the same cell and stretch to fill it. No absolute positioning, no z-index required (though you can still use it if you need to reorder).

This is often the cleaner choice when you want the overlap to participate in normal layout flow — for example, when the container's size should be driven by its children's intrinsic size.

When to Use Which

Situation	Reach for
Overlaying a loading spinner, tooltip, or tag on a fixed-size box	Absolute positioning
Layering content where the tallest child should define the container height	CSS Grid
You already have a position: relative parent for other reasons	Absolute positioning
You want the most modern, least-surprising solution	CSS Grid

Conclusion

Two tools, one problem. Absolute positioning is the battle-tested approach every frontend developer should know. CSS Grid's single-cell trick is the modern alternative that keeps children in the layout flow.

Next time you reach for position: absolute out of habit, ask yourself whether grid-area: 1 / 1 would be cleaner.