:first-child

p {
  font-weight: bold;
}

li:first-child {
  border: 2px solid orange;
}

<p>Track &amp; field champions:</p>
<ul>
  <li>Adhemar da Silva</li>
  <li>Wang Junxia</li>
  <li>Wilma Rudolph</li>
  <li>Babe Didrikson-Zaharias</li>
  <li>Betty Cuthbert</li>
  <li>Fanny Blankers-Koen</li>
  <li>Florence Griffith-Joyner</li>
  <li>Irena Szewinska</li>
  <li>Jackie Joyner-Kersee</li>
  <li>Shirley Strickland</li>
  <li>Carl Lewis</li>
  <li>Emil Zatopek</li>
  <li>Haile Gebrselassie</li>
  <li>Jesse Owens</li>
  <li>Jim Thorpe</li>
  <li>Paavo Nurmi</li>
  <li>Sergei Bubka</li>
  <li>Usain Bolt</li>
</ul>

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

<div>
  <p>此文本已选中！</p>
  <p>此文本未选中。</p>
</div>

<div>
  <h2>此文本未被选中：它不是一个 `p`。</h2>
  <p>此文本未被选中。</p>
</div>

p:first-child {
  color: lime;
  background-color: black;
  padding: 5px;
}

<ul>
  <li>Item 1</li>
  <li>Item 2</li>
  <li>
    Item 3
    <ul>
      <li>Item 3.1</li>
      <li>Item 3.2</li>
      <li>Item 3.3</li>
    </ul>
  </li>
</ul>

ul li {
  color: blue;
}

ul li:first-child {
  color: red;
  font-weight: bold;
}