Element: focusin イベント

addEventListener("focusin", (event) => {});

<form id="form">
  <label>
    テキストを入力:
    <input type="text" placeholder="テキストを入力" />
  </label>
  <label>
    パスワード:
    <input type="password" placeholder="password" />
  </label>
</form>

const form = document.getElementById("form");
form.addEventListener("focusin", (event) => {
  event.target.style.background = "pink";
});
form.addEventListener("focusout", (event) => {
  event.target.style.background = "";
});