If we want to select one or more elements, we can set a class attribute for the exact elements we want to change.
In the stylesheet, we can set a new class rule with a . followed by the name of the class. Like here with .gray-element.
<head>
<link rel="stylesheet" href="style.css">
</head>
<body>
<ol> TO DO: <li class="gray-element">Floss</li>
<li>Feed Cat</li>
<li class="gray-element">Nap</li>
<li>Plan holiday</li>
<li class="gray-element">Order tea</li>
</ol>
</body>
.gray-element {
background-color: lightgrey;
}