<div class="responsive-box"></div>
<style>
.responsive-box {
width: 50vw;
height: 50vw;
background-color: green;
animation: resize 4s infinite;
}
@keyframes resize {
0% { width: 50vw; height: 50vw; }
50% { width: 70vw; height: 70vw; }
100% { width: 50vw; height: 50vw; }
}
</style>