: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>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;
}

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