<div>
  <div id="msg">HELLO</div>
</div>

<script>
let x = document.body.clientWidth;
  
setInterval(()=>{
    document.getElementById('msg').setAttribute('style','position:relative;left:'+x+'px;');
    x-=10;
    if(x<=0)
      x=document.body.clientWidth;
},100
);
 
</script>