HTMLElement: autocapitalize プロパティ

<div>現在の大文字化の動作は <span id="ac-label"></span> です。</div>
<div id="ac-element" contenteditable="true" autocapitalize="default">
  input here
</div>
<select id="ac-controller" type="checkbox" checked>
  <option value="default">既定値</option>
  <option value="none">なし</option>
  <option value="sentences">文</option>
  <option value="words">単語</option>
  <option value="characters">文字</option></select
>大文字化の動作を選択してください。

const label = document.getElementById("ac-label");
const element = document.getElementById("ac-element");
const controller = document.getElementById("ac-controller");
controller.addEventListener("input", (e) => {
  const behavior = e.target.value;
  label.textContent = behavior;
  element.autocapitalize = behavior;
});