#test1 { background: red; display: block; padding: 1em; margin: 1em; }
   #test1:empty { background: lime; }
   #test2 { background: lime; display: block; padding: 1em; margin: 1em; }
   #test2:empty { background: red; }

 <div>

  <script type="text/javascript">
   

    function test() {
      document.getElementById('test1').appendChild(document.createTextNode(''));
      document.getElementById('test2').appendChild(document.createTextNode(' '));
    }

    window.setTimeout("test()", 100);
   
  </script>

  <p> The following two bars should be green. </p>

  <div id="test1"></div>
  <div id="test2"></div>

 </div>