/*css文件*/

/* 遮罩层 */
#modal-overlay {
  visibility: hidden;
  position: absolute;
  left: 0px;  /*如果 left和top不设置为0，边框会有白边 */
  top: 0px;
  width: 100%;
  height: 100%;
  text-align: center;
  z-index: 1000;
  backgroun-color: #333;
  opacity: 0.5;
}

/*内容*/
.modal-data {
  width: 300px;
  margin: 100px auto;
  background-color: #fff;
  border: 1px solid #000;
  padding: 15px;
  text-align: center;
}

<!-- html内容-->
<div id="modal-overlay">
    <div class="modal-data">
      <p>简单的模态对话框</p>
      <p>点击<a href="javascript:overlay()">关闭</a></p>
    </div>
  </div>
  <button onclick="showModal()">显示模态对话框</button>

//js代码
function overlay() {
    var el = document.getElementById('modal-overlay');
    el.style.visibility = (el.style.visibility == "visible") ? "hidden" : "visible";
  }

  function showModal() {
    var el = document.getElementById('modal-overlay');
    el.style.visibility = "visible";
  }

模态框的美化

<!-- html代码 -->
<!-- 遮罩层 -->
<div id="openModal" class="modalDialog">
    <div>  <!-- 对话框 -->
        <a href="#close" title="Close" class="close">X</a> <!-- 关闭按钮-->
        <h2>Modal Box</h2>
        <p>This is a sample modal box that can be created using the power of CSS3.</p>
        <p>You could do a lot of things here like have a pop-up ad that shows when your website loads, or create a login/register form for users.</p>
    </div>
</div>

/* css代码 */
.modalDialog {
  position: fixed;
  font-family: Arial, Helvetica, sans-serif;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color; rgba(0, 0, 0, 0.8);
  z-index: 9999;
  opacity: 0;      /*初始未显示，opacity: 1;显示*/
  -webkit-transition: opacity 400ms ease-in;
  -moz-transition: opacity 400ms ease-in;
  transition: opacity 400ms ease-in;
  /* ease:逐渐慢下来；linear:匀速；ease-in:由慢到快；ease-out:由快到慢；ease-in-out:先慢到快再到慢*/
  pointer-events: none; /*元素永远不会成为鼠标事件的target。*/
}

.modalDialog:target {
    opacity: 1;
}

.modalDialog > div {
    width: 400px;
    position: relative;
    margin: 10% auto;
    padding: 5px 20px 13px 20px;
    border-radius: 10px;
    background: #fff;
    background: -webkit-linear-gradient(#fff, #999);
    background: -o-linear-gradient(#fff, #999);
    background: -moz-linear-gradient(#fff, #999);
}

.close {
    background: #606061;
    color: #FFFFFF;
    line-height: 25px;
    position: absolute;
    right: -12px;
    text-align: center;
    top: -10px;
    width: 24px;
    text-decoration: none;
    font-weight: bold;
    border-radius: 50%;
    box-shadow: 1px 1px 3px #000;
    pointer-events: auto;
}

.close:hover {
    background: #00d9ff;
}