:first-child

<div>
  <span>This span is limed!</span>
  <span>This span is not. :(</span>
</div>

span:first-child {
    background-color: lime;
}

<ul>
  <li>List 1</li>
  <li>List 2</li>
  <li>List 3</li>
</ul>

li{
  color:red;
}
li:first-child{
  color:green;
}