使用animation-timing-function属性，并使用easy值将CSS的动画设置从慢速开始，然后从快速开始，以CSS缓慢结束
<!DOCTYPE html>
<html>
<head>
<style>
div {
width: 150px;
height: 200px;
position: relative;
background-color: yellow;
animation-name: myanim;
animation-duration: 2s;
animation-direction: alternate-reverse;
animation-iteration-count: 3;
}
@keyframes myanim {
from {left: 100px;}
to {left: 200px;}
}
#demo1 {animation-timing-function: ease;}
</style>
</head>
<body>
<div id = "demo1">ease effect</div>
</body>
</html>