效果：
源代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
div{
margin-top: 10px;
height: 50px;
background-color: #FF0000;
opacity: 0.6;
}
.a{
animation: aa 2s linear 100ms infinite;
}
.b{
animation: bb 2s linear infinite
}
.c{
animation: cc 2s linear infinite
}
.d{
animation:dd 2s linear infinite
}
@keyframes aa{
0%{width: 0;}
25%{width:200px;}
50%{width:200px;}
75%{width:200px;}
100%{width:200px;}
}
@keyframes bb{
0%{width: 0;}
25%{width:0px;}
50%{width:200px;}
75%{width:200px;}
100%{width:200px;}
}
@keyframes cc{
0%{width: 0;}
25%{width:0px;}
50%{width:0px;}
75%{width:200px;}
100%{width:200px;}
}
@keyframes dd{
0%{width: 0;}
25%{width:0px;}
50%{width:0px;}
75%{width:0px;}
100%{width:200px;}
}
</style>
</head>
<body>
<div class="a">
</div>
<div class="b">
</div>
<div class="c">
</div>
<div class="d">
</div>
</body>
</html>