dialog::backdrop {
  background-color: rgba(0, 0, 0, 0);
  transition: background-color 0.3s ease;
}

dialog::backdrop {
  animation: backdrop-fade-in 0.3s ease forwards;
}

@keyframes backdrop-fade-in {
  from {
    background-color: rgba(0, 0, 0, 0);
  }
  to {
    background-color: rgba(0, 0, 0, 0.5);
  }
}

dialog[open]::backdrop {
  animation: backdrop-fade-out 0.3s ease forwards;
}

@keyframes backdrop-fade-out {
  from {
    background-color: rgba(0, 0, 0, 0.5);
  }
  to {
    background-color: rgba(0, 0, 0, 0);
  }
}

<dialog id="myDialog">
  <p>這是一個模態(tài)框</p>
  <button id="closeBtn">關(guān)閉</button>
</dialog>
<button id="openBtn">打開模態(tài)框</button>

<script>
  const dialog = document.getElementById('myDialog');
  const openBtn = document.getElementById('openBtn');
  const closeBtn = document.getElementById('closeBtn');

  openBtn.addEventListener('click', () => {
    dialog.showModal(); // 觸發(fā) ::backdrop
  });

  closeBtn.addEventListener('click', () => {
    dialog.close();
  });
</script>

dialog::backdrop {
  background-color: rgba(0, 0, 0, 0.4);
  backdrop-filter: blur(4px);
  transition: all 0.3s ease;
}