<style>
.box {
width: 100px;
height: 100px;
background-color: #9b59b6;
animation: move 4s infinite;
animation-direction: alternate-reverse;
}
@keyframes move {
0% { transform: translateX(0); }
100% { transform: translateX(200px); }
}
</style>
</head>
<body>
<div class="box"></div>