<form id="form">
  <input type="text" name="surname" value="성">
  <input type="text" name="name" value="이름">
</form>

<style> .focused { outline: 1px solid red; } </style>

<script>
  // 캡쳐링 단계에서 핸들러가 트리거되도록 함(마지막 인수가 true)
  form.addEventListener("focus", () => form.classList.add('focused'), true);
  form.addEventListener("blur", () => form.classList.remove('focused'), true);
</script>