HTMLElement: dragstart イベント

addEventListener("dragstart", (event) => {});
ondragstart = (event) => {};

<div id="container">
  <div id="draggable" draggable="true">この div はドラッグ可</div>
</div>
<div class="dropzone"></div>

body {
  /* 例でユーザーがテキストを選択するのを防ぐ */
  user-select: none;
}
#draggable {
  text-align: center;
  background: white;
}
#container {
  width: 200px;
  height: 20px;
  background: blueviolet;
  padding: 10px;
}
.dragging {
  opacity: 0.5;
}

const source = document.getElementById("draggable");
source.addEventListener("dragstart", (event) => {
  // 半透明にする
  event.target.classList.add("dragging");
});
source.addEventListener("dragend", (event) => {
  // 透明度を解除
  event.target.classList.remove("dragging");
});