Add a class attribute to the li element.

<html>
  <head>
   <style>
.redElement {
background-color: red;}
   </style>
  </head>
<body>
<ol>TO DO:
  <li class="redElement">Exercise</li>
    <li>Groom dog</li>
    <li class="redElement">Stretch</li>
    <li>Tidy apartment</li>
    <li class="redElement">Laundry</li>
</ol>
</body>
</html>]]>

Make sure to assemble class = "redElement".