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Stacking without the z-index property

When the z-index property is not specified on any element, elements are stacked in the following order (from bottom to top):

The background and borders of the root element.
Descendant non-positioned elements, in order of appearance in the HTML.
Descendant positioned elements, in order of appearance in the HTML.

See types of positioning for an explanation of positioned and non-positioned elements.

Keep in mind, when the order property alters rendering from the order of appearance in the HTML within flex containers, it similarly affects the order for stacking context.

Example

In this example, DIV #1 through DIV #4 are positioned elements. DIV #5 is static, and so is drawn below the other four elements, even though it comes later in the HTML markup.

HTML

<div id="abs1" class="absolute">
  <strong>DIV #1</strong><br />position: absolute;
</div>
<div id="rel1" class="relative">
  <strong>DIV #2</strong><br />position: relative;
</div>
<div id="rel2" class="relative">
  <strong>DIV #3</strong><br />position: relative;
</div>
<div id="abs2" class="absolute">
  <strong>DIV #4</strong><br />position: absolute;
</div>
<div id="sta1" class="static">
  <strong>DIV #5</strong><br />position: static;
</div>

CSS

strong {
  font-family: sans-serif;
}

div {
  padding: 10px;
  border: 1px dashed;
  text-align: center;
}

.static {
  position: static;
  height: 80px;
  background-color: #ffc;
  border-color: #996;
}

.absolute {
  position: absolute;
  width: 150px;
  height: 350px;
  background-color: #fdd;
  border-color: #900;
  opacity: 0.7;
}

.relative {
  position: relative;
  height: 80px;
  background-color: #cfc;
  border-color: #696;
  opacity: 0.7;
}

#abs1 {
  top: 10px;
  left: 10px;
}

#rel1 {
  top: 30px;
  margin: 0px 50px 0px 50px;
}

#rel2 {
  top: 15px;
  left: 20px;
  margin: 0px 50px 0px 50px;
}

#abs2 {
  top: 10px;
  right: 10px;
}

#sta1 {
  background-color: #ffc;
  margin: 0px 50px 0px 50px;
}

W3cubDocs

Stacking without the z-index property

Example

HTML

CSS

Result

See also