<button id="button-open">モーダルを開く</button>
<dialog id="my-modal">
  <p>モーダルの中身です。</p>
  <button id="button-close">モーダルを閉じる</button>
</dialog>

#my-modal:modal {
  opacity: 0;
}
#my-modal:modal {
  animation: fadeIn .5s forwards;
}
#my-modal:modal.is-hide {
  animation: fadeOut .5s forwards;
}

#my-modal::backdrop {
  opacity: 0;
  background-color: rgba(0,0,0,.8);
}
#my-modal::backdrop {
  animation: fadeIn .5s forwards;
}
#my-modal.is-hide::backdrop {
  animation: fadeOut .5s forwards;
}

@keyframes fadeIn {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

@keyframes fadeOut {
  from {
    opacity: 1;
  }
  to {
    opacity: 0;
  }
}

const modal = document.querySelector('#my-modal')
const buttonOpen = document.querySelector('#button-open')
const buttonClose = document.querySelector('#button-close')

buttonOpen.addEventListener('click', () => {
  modal.showModal()
})

buttonClose.addEventListener('click', () => {
  modal.classList.add('is-hide')
  modal.addEventListener('webkitAnimationEnd', function(){
    modal.classList.remove('is-hide')
    modal.close()
 
    modal.removeEventListener('webkitAnimationEnd', arguments.callee, false);
  }, false);
})