<form>
  <input type="text">
  <br>input을 클릭하면 form 영역의 색이 바뀌어요.
</form>
form:focus-within {
    /*form 태그 내부에 focus된 요소가 있으면 선택자가 적용됩니다.*/
    background-color:red;
}

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