Untitled

<!DOCTYPE html>
  <head>
    <style>
    span.red{color:red;}
    span.blue{color:blue;}
    </style>
  </head>
  <body>
  <span id="demo"></span>
  <script>
    document.write("Var a:");
  var a = window.prompt();
  var obj = document.getElementById('demo');

  obj.innerHTML = a;

  if(a > 0){ obj.className = "red";}
   if(a < 0){ obj.className = "blue";}
  </script>
</body>