<div class="list">
  <p>変わりません</p>
  <p>変わりません</p>
  <p>変わりません</p>
  <p>変わりません</p>
  <p>最後の要素が赤色に変わります。</p>
</div>
.list p:last-child {
  color: #f00;
}

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