#test { background: red; display: block; padding: 1em; }
   #test:not(:empty) { background: lime; }

 <div>

  <script>
   

    function test() {
      document.getElementById('test').appendChild(document.createTextNode('This line should be green.'));
    }

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

  <p> The following bar should be green. </p>

  <div id="test"></div>

 </div>