:only-child

:only-child {
  /* ... */
}

<div>
  <div>I am an only child.</div>
</div>

<div>
  <div>I am the 1st sibling.</div>
  <div>I am the 2nd sibling.</div>
  <div>
    I am the 3rd sibling,
    <div>but this is an only child.</div>
  </div>
</div>

div:only-child {
  color: red;
}

div {
  display: inline-block;
  margin: 6px;
  outline: 1px solid;
}

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

li li {
  list-style-type: disc;
}

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