<button id="openModal">Open modal</button>

<!-- モーダルエリアここから -->
<section id="modalArea" class="modalArea">
  <div id="modalBg" class="modalBg"></div>
  <div class="modalWrapper">
    <div class="modalContents">
      <h1>Here are modal without jQuery!</h1>
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. </p>
    </div>
    <div id="closeModal" class="closeModal">
      ×
    </div>
  </div>
</section>
<!-- モーダルエリアここまで -->

* {
  box-sizing: border-box;
}
body {
  font-family:'Avenir','Helvetica, Neue','Helvetica','Arial';
  height: 5000px;
}


/* モーダルCSSここから */
.modalArea {
  visibility: hidden; /* displayではなくvisibility */
  opacity : 0;
  position: fixed;
  z-index: 10; /* サイトによってここの数値は調整 */
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  transition: .4s;
}

.modalBg {
  width: 100%;
  height: 100%;
  background-color: rgba(30,30,30,0.9);
}

.modalWrapper {
  position: absolute;
  top: 50%;
  left: 50%;
  transform:translate(-50%,-50%);
  width: 70%;
  max-width: 500px;
  padding: 10px 30px;
  background-color: #fff;
}

.closeModal {
  position: absolute;
  top: 0.5rem;
  right: 1rem;
  cursor: pointer;
}

.is-show { /* モーダル表示用クラス */
  visibility: visible;
  opacity : 1;
}
/* モーダルCSSここまで */


/* 以下ボタンスタイル */
button {
  padding: 10px;
  background-color: #fff;
  border: 1px solid #282828;
  border-radius: 2px;
  cursor: pointer;
}

#openModal {
  position: absolute;
  top: 50%;
  left: 50%;
  transform:translate(-50%,-50%);
}

(function () {
  const modalArea = document.getElementById('modalArea');
  const openModal = document.getElementById('openModal');
  const closeModal = document.getElementById('closeModal');
  const modalBg = document.getElementById('modalBg');
  const toggle = [openModal,closeModal,modalBg];
  
  for(let i=0, len=toggle.length ; i<len ; i++){
    toggle[i].addEventListener('click',function(){
      modalArea.classList.toggle('is-show');
    },false);
  }
}());