<input type="button" value="click" id="btn">
<div class="modal" id="modal">
  <div class="dialog">
    <header class="dialog-header">
      <h3>this is dialog title</h3>
      <span id="close">×</span>
    </header>
    <div class="dialog-content">
      this is dialog content
     </div>
   </div>
</div>

.modal{
    position:fixed;
    left:0;
    right:0;
    top:0;
    bottom:0;
    background-color:rgba(0,0,0,0.5);
    display:none;
    z-index:1050;
    opacity:0;
    transition: all .5s ease-out 0s;
  }
  .dialog{
    width:500px;
    height:300px;
    position:relative;
    box-shadow:0 5px 15px rgba(0,0,0,.5);
    border-radius:10px;
    background-color:#fff;
    margin:30px auto;
    transform: translate(0,-40%);
    -webkit-transform: translate(0,-40%);
    transition: all .5s ease-out 0s;
  }
  .dialog-header{
    box-sizing:border-box;
    border-bottom:1px solid #ccc;
  }
  .dialog-header h3,.dialog-header span{
    display:inline-block;
  }
  .dialog-header span{
    float:right;
    margin-right:10px;
    overflow: hidden;
    line-height:58px;
    cursor:default;
    font-size:18px;
  }
  .in{
    opacity: 1;
  }
  .in .dialog{
    transform: translate(0,0);
    -webkit-transform: translate(0,0);
  }

var oBtn = document.getElementById("btn");
var oModal = document.getElementById("modal");
var oClose = document.getElementById("close");
oBtn.addEventListener("click", function(){
  oModal.style.display = "block";
  oModal.className = "modal in";
});
oClose.addEventListener("click", function(){
  oModal.style.display = "none";
  oModal.className = "modal";
});

  oModal.style.display = "block";
  var timer = setTimeout(function(){
    oModal.className = "modal in";
    clearTimeout(timer);
  },0);