Edit in JSFiddle

TweenMax.to(document.getElementById("box"), 3, {
    bezier:[
        {left:100, top:250},
        {left:200, top:0},
        {left:300, top:100}
    ], ease:Power1.easeInOut, repeat: -1, yoyo: true
});

<div id="box">
  <span class="inner front">앞</span>
  <span class="inner back">뒤</span>
</div>

#box{
    position: absolute;
    top: 0;left: 0;
    width: 50px;
    height: 50px;
    background-color: cornflowerblue;
    border: 1px solid #000;
}
.inner{
    position: absolute;
    left: 0;
    width: 50px;
    height: 22px;
    background-color: honeydew;
    text-align: center;
}
.inner.front{top: 0;}
.inner.back{bottom: 0;}