:only-child

/* Selects each <p>, but only if it is the */
/* only child of its parent */
p:only-child {
  background-color: lime;
}

:only-child

<main>
  <div>
    <i>I am a lonely only child.</i>
  </div>

  <div>
    <i>I have siblings.</i><br>
    <b>So do I!</b><br>
    <span>I also have siblings, <span>but this is an only child.</span></span>
  </div>
</main>

main :only-child {
  color: red;
}

<ol>
  <li>First
    <ul>
      <li>This list has just one element.
    </ul>
  </li>
  <li>Second
    <ul>
      <li>This list has three elements.
      <li>This list has three elements.
      <li>This list has three elements.
    </ul>
  </li>
<ol>

li li {
  list-style-type: disc;
}
li:only-child {
  color: red;
  list-style-type: square;
}