<dialog id="modal">
  	<p>Hi! This is a modal.</p>
  	<button id="hide-modal">OK</button>
</dialog>
<button id="show-modal">
  Show Modal
</button>
<script>
#modal {
  display: block;
  opacity: 0;
  pointer-events: none;
  border: none;
  top: 50px;
}

#modal[open] {
  opacity: 1;
  pointer-events: auto;
  animation: slidein 0.5s forwards;
}

#modal::backdrop {
  background-color: rgba(0, 0, 0, 0.6);
  animation: fadein 0.5s forwards;
}

#modal.close[open] {
  animation: slideout 0.5s forwards;
}

#modal.close::backdrop {
  animation: fadeout 0.5s forwards;
}

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

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

@keyframes slidein {
  from {
    transform: translateY(-100%);
    opacity: 0;
  }
  to {
    transform: translateY(0);
    opacity: 1;
  }
}

@keyframes slideout {
  from {
    transform: translateY(0);
    opacity: 1;
  }
  to {
    transform: translateY(-100%);
    opacity: 0;
  }
}
const modal = document.getElementById("modal");
const showModalBtn = document.getElementById("show-modal");
const hideModalBtn = document.getElementById("hide-modal");

function closeModal() {
  modal.close();
  modal.classList.remove('close');
  modal.removeEventListener('animationend', closeModal);
}

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

hideModalBtn.addEventListener('click', () => {
  modal.addEventListener('animationend', closeModal);
  modal.classList.add('close');
});

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.