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

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

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.

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

Source code for the example

HTML

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

CSS

b {
  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;
}

See also

Original Document Information