:only-of-type

a:only-of-type {
  color: fuchsia;
}
dd:only-of-type {
  background-color: bisque;
}

<p>
  To find out more about <b>QUIC</b>, check <a href="#">RFC 9000</a> and
  <a href="#">RFC 9114</a>.
</p>
<dl>
  <dt>Published</dt>
  <dd>2021</dd>
  <dd>2022</dd>
</dl>
<p>Details about <b>QPACK</b> can be found in <a href="#">RFC 9204</a>.</p>
<dl>
  <dt>Published</dt>
  <dd>2022</dd>
</dl>

<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;
}