现场演示：我的 HTML 文档

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Example of CSS z-index Property Animation</title>
<style>
.container{
	position: relative;
}
.container div{
	width: 150px;
    height: 150px;
	padding: 5px;
	position: absolute;
	background: #76bf40;
	border: 1px solid #000;
	opacity: 0.3;
}
.container .animated {
	z-index: 0;
    background: #ffd700;	
    border: 1px solid #b79b00;
	opacity: 1;
    -webkit-animation: test 4s infinite; /* Chrome, Safari, Opera */
    animation: test 4s infinite;
}
/* Chrome, Safari, Opera */
@-webkit-keyframes test {
    50% {z-index: 4;}
}
/* Standard syntax */
@keyframes test {
    50% {z-index: 4;}
}
</style>
</head>
<body>
	<p><strong>Warning:</strong> CSS Animations do not work in Internet Explorer 9 and earlier versions.</p>
	<p><strong>Note:</strong> The position of the animated DIV box along the z-axis is animating from its initial value "0" to "4", and back to the initial value "0" again up to infinite times.<p>
	<div class="container">
		<div class="animated">Animated Box</div>
		<div style="top:30px; left:30px; z-index:1;">z-index 1</div>
		<div style="top:60px; left:60px; z-index:2;">z-index 2</div>
		<div style="top:90px; left:90px; z-index:3;">z-index 3</div>
	</div>
</body>
</html>

xxxxxxxxxx
 
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Example of CSS z-index Property Animation</title>
<style>
.container{
    position: relative;
}
.container div{
    width: 150px;
    height: 150px;
    padding: 5px;
    position: absolute;
    background: #76bf40;
    border: 1px solid #000;
    opacity: 0.3;
}
.container .animated {
    z-index: 0;
    background: #ffd700;    
    border: 1px solid #b79b00;
    opacity: 1;
    -webkit-animation: test 4s infinite; /* Chrome, Safari, Opera */
    animation: test 4s infinite;
}
/* Chrome, Safari, Opera */
@-webkit-keyframes test {
    50% {z-index: 4;}
}
/* Standard syntax */
@keyframes test {
    50% {z-index: 4;}
}
</style>
</head>
<body>
    <p><strong>Warning:</strong> CSS Animations do not work in Internet Explorer 9 and earlier versions.</p>
    <p><strong>Note:</strong> The position of the animated DIV box along the z-axis is animating from its initial value "0" to "4", and back to the initial value "0" again up to infinite times.<p>
    <div class="container">
        <div class="animated">Animated Box</div>
        <div style="top:30px; left:30px; z-index:1;">z-index 1</div>