red and black

<script type="text/javascript">
window.onload=function() {
 var isRed = false;
 var selectAll = document.body;
 setInterval(function() {
  if (!isRed) {
         selectAll.style.color = "red";
         isRed = true;
     } else {
      selectAll.style.color = "black";
      isRed = false;
     }
    }, 100);
}
</script>