z-index

z-index: auto;

z-index: 1;

z-index: 3;

z-index: 5;

z-index: 7;

<section class="default-example container" id="default-example">
  <div id="example-element">Change my z-index</div>
  <div class="block blue position1">z-index: 6</div>
  <div class="block blue position2">z-index: 4</div>
  <div class="block blue position3">z-index: 2</div>
  <div class="block red position4">z-index: auto</div>
  <div class="block red position5">z-index: auto</div>
  <div class="block red position6">z-index: auto</div>
</section>

#example-element {
  top: 15px;
  left: 15px;
  width: 180px;
  height: 230px;
  position: absolute;
  /* center the text so it is visible even when z-index is set to auto */
  line-height: 215px;
  font-family: monospace;
  background-color: #fcfbe5;
  border: solid 5px #e3e0a1;
  z-index: auto;
  color: black;
}
.container {
  display: inline-block;
  width: 250px;
  position: relative;
}
.block {
  width: 150px;
  height: 50px;
  position: absolute;
  font-family: monospace;
  color: black;
}
.blue {
  background-color: #e5e8fc;
  border: solid 5px #112382;
  /* move text to the bottom of the box */
  line-height: 55px;
}
.red {
  background-color: #fce5e7;
  border: solid 5px #e3a1a7;
}
.position1 {
  top: 0;
  left: 0;
  z-index: 6;
}
.position2 {
  top: 30px;
  left: 30px;
  z-index: 4;
}
.position3 {
  top: 60px;
  left: 60px;
  z-index: 2;
}
.position4 {
  top: 150px;
  left: 0;
  z-index: auto;
}
.position5 {
  top: 180px;
  left: 30px;
  z-index: auto;
}
.position6 {
  top: 210px;
  left: 60px;
  z-index: auto;
}

/* キーワード値 */
z-index: auto;
/* <integer> 値 */
z-index: 0;
z-index: 3;
z-index: 289;
z-index: -1; /* 負の数はより低い優先度になる */
/* グローバル値 */
z-index: inherit;
z-index: initial;
z-index: revert;
z-index: revert-layer;
z-index: unset;

<div class="wrapper">
  <div class="dashed-box">破線のボックス</div>
  <div class="gold-box">金色のボックス</div>
  <div class="green-box">緑色のボックス</div>
</div>

.wrapper {
  position: relative;
}
.dashed-box {
  position: relative;
  z-index: 1;
  border: dashed;
  height: 8em;
  margin-bottom: 1em;
  margin-top: 2em;
}
.gold-box {
  position: absolute;
  z-index: 3; /* .gold-box を .green-box や .dashed-box の上に置く */
  background: gold;
  width: 80%;
  left: 60px;
  top: 3em;
}
.green-box {
  position: absolute;
  z-index: 2; /* .green-box を .dashed-box の上に置く */
  background: lightgreen;
  width: 20%;
  left: 65%;
  top: -25px;
  height: 7em;
  opacity: 0.9;
}