<div class="flex">
  <div class="w20">1</div>
  <div class="w20">2</div>
  <div class="w20 berth">3</div>
  <div class="w20 berth">4</div>
  <div class="w20 berth">5</div>
</div>
.first {
  color: red;
}
const berths = document.querySelectorAll('.flex .berth');
berths[0].classList.add('first');

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External JavaScript

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