index.html

<style>
  #a, #b {
    position: absolute;
    top: 0;
    left: 0;
    width: 20%;
    height: 100px;
    text-align: center;
    line-height: 100px;
    background: red;
    overflow: hidden;
  }

  #b {
    right: 0;
    left: initial;
    background: green;
  }
</style>

<div id="a">A</div>
<div id="b">1</div>

<script>
  setInterval(function(){
    var val = ++b.textContent
    b.textContent = val % 10
  }, 1000)
</script>