:first-of-type

/* 兄弟要素の中で最初の <p> 要素をすべて選択 */
p:first-of-type {
  color: red;
}

<h2>Heading</h2>
<p>Paragraph 1</p>
<p>Paragraph 2</p>

p:first-of-type {
  color: red;
  font-style: italic;
}

<article>
  <div>This `div` is first!</div>
  <div>This <span>nested `span` is first</span>!</div>
  <div>This <em>nested `em` is first</em>, but this <em>nested `em` is last</em>!</div>
  <div>This <span>nested `span` gets styled</span>!</div>
  <b>This `b` qualifies!</b>
  <div>This is the final `div`.</div>
</article>

article :first-of-type {
  background-color: pink;
}