<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 {
  padding: 2.5rem;
  border: 0;
  border-radius: .5rem;
  background-color: #fff;
  box-shadow: 0 0 2rem .5rem rgba(0,0,0,.1);
}
#my-modal:modal p {
  margin: .5rem 0 0;
  color: #333;
  font-size: 1.2rem;
  line-height: 1.2;
  font-weight: bold;
  letter-spacing: .025em;
  text-align: center;
}
#my-modal:modal button {
  display: block;
  width: 100%;
  margin-top: 1rem;
  border: 0;
  padding: .9rem;
  border-radius: .5rem;
  background-color: dodgerblue;
  color: white;
  font-size: 1rem;
  line-height: 1.2;
  font-weight: bold;
  letter-spacing: .025em;
}
#my-modal::backdrop {
  background-color: rgba(255,255,255,.8);
}

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.close()
})