Untitled


<!DOCTYPE html>
<html>
<head>
<style type="text/css">
.container {
  visibility: hidden;
  opacity: 0;
  background-color: rgba(0, 0, 0, .5);
  position: absolute;
  height: 100%;
  width: 100%;
  top: 0;
  left: 0;
  display: flex;
}

.modal {
  margin: auto;
  z-index: 999;
  background-color: blue;
  width: 300px;
  height: 100px;
}

.displayed {
  opacity: 1;
  transition: opacity 0.8s;
  visibility: visible;
}
</style>

<script type='text/javascript'>
window.onload=function(){
  function showModal() {
    var modal = document.getElementById('modal');
    modal.className += " displayed";
  }

  document.getElementById('button').addEventListener('click', showModal);
}
</script>

</head>

<body>
  <div>
  content
  <button id="button">
    show modal
  </button>
  <div id="modal" class="container">

    <div class="modal">
      modal content
    </div>
  </div>
</div>
</body>
</html>