:only-child

li:only-child {
  color: fuchsia;
}

b:only-child {
  text-decoration: underline;
}

<p>Stars expected to attend:</p>
<ol>
  <li>Robert Downey, Jr.</li>
</ol>

<p>Stars yet to confirm:</p>
<ol>
  <li>Scarlett Johansson</li>
  <li>Samuel L. Jackson</li>
  <li>Chris Pratt</li>
</ol>

<p>The ceremony is going to be held in <b>The Dolby Theatre</b>.</p>

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

<div>
  <div>我是唯一子元素。</div>
</div>

<div>
  <div>我是第一个兄弟元素。</div>
  <div>我是第二个兄弟元素。</div>
  <div>
    我是第三个兄弟元素。
    <div>但这又是唯一的子元素。</div>
  </div>
</div>

div:only-child {
  color: red;
}

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

<ol>
  <li>
    第一
    <ul>
      <li>该列表仅有一个元素。</li>
    </ul>
  </li>
  <li>
    第二
    <ul>
      <li>该列表有三个元素。</li>
      <li>该列表有三个元素。</li>
      <li>该列表有三个元素。</li>
    </ul>
  </li>
</ol>

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

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