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                <div class="contents">

<div class="container">
  <button class="modal-btn modal-open" type="button" id="modalOpen">Open</button>

<div id="modal" class="modal">
  <div class="modal-overlay js-modal-close">
    <div class="modal-container">
      <div class="modal-content">
        <h2 class="modal-content-ttl">モーダルのタイトルです</h2>
        <p class="modal-content-txt">Lorem ipsum, dolor sit amet consectetur adipisicing elit. Quibusdam dignissimos delectus quia ducimus quae architecto repellat expedita rem ipsam quisquam esse, cum atque</p>
      </div><!-- /.modal-content -->
      <button class="modal-btn modal-close js-modal-close">Close</button>
    </div><!-- /.modal-container -->
  </div><!-- /.modal-overlay -->
</div><!-- /.modal -->
</div><!-- /.contents -->


                * {
  margin: 0;
  padding: 0;

  font-family: 'Helvetica Neue', Arial, 'Hiragino Kaku Gothic Pro N', Meiryo, sans-serif;

  max-width: 400px;
  padding: 80px 40px 0 40px;
  margin: 0 auto;

.modal-btn {
  border: none;
  border-radius: 4px;
  color: #fff;
  cursor: pointer;
  font-size: 1rem;
  padding: 10px 20px;

  background: orangered;

  background: darkblue;

.modal {
  display: none;

.modal.is_open {
  display: block;

.modal-overlay {
  align-items: center;
  background: rgba(0, 0, 0, .75);
  bottom: 0;
  display: flex;
  justify-content: center;
  left: 0;
  position: fixed;
  right: 0;
  top: 0;

.modal-container {
  background: #fff;
  border-radius: 4px;
  max-height: 100%;
  max-width: 600px;
  padding: 30px 20px;
  overflow: scroll;

  font-size: 2rem;

  line-height: 1.67;


                const modalBtn = document.getElementById('modalOpen');
const modal = document.getElementById('modal');

const ms = 400; = 'opacity ' + ms + 'ms';

modalBtn.addEventListener('click', () =>{
  setTimeout(function(){ = 1}, 50);

let modalClose = document.querySelectorAll('.js-modal-close');
  for (let i = 0; i < modalClose.length; i++){
  modalClose[i].addEventListener('click', () => {
    setTimeout(function(){ = 0}, 1);
    setTimeout(function(){modal.classList.remove('is_open')}, ms)