:only-of-type

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

:only-of-type

<main>
  <div>I am `div` #1.</div>
  <p>I am the only `p` among my siblings.</p>
  <div>I am `div` #2.</div>
  <div>I am `div` #3.
    <i>I am the only `i` child.</i>
    <em>I am `em` #1.</em>
    <em>I am `em` #2.</em>
  </div>
</main>

main :only-of-type {
  color: red;
}