getComputedStyle获取css属性与IE下的currentStyle获取到的值不同

<!doctype html>
<html lang="en">
 <head>
  <meta charset="UTF-8">
  <meta name="Generator" content="EditPlus®">
  <meta name="Author" content="">
  <meta name="Keywords" content="">
  <meta name="Description" content="">
  <title>Document</title>
  <style>
    .button {
    height: 2em;
    line-height:24px;
    //border: 0;
    //border-radius: .2em;
    background-color: #34538b;
    color: #fff;
    font-size: 12px;
    font-weight: bold;
    }
    </style>
 </head>

<body>
      <input type="button" id="button" class="button" value="点击我，显示高度" />
    <script>
        document.getElementById("button").onclick = function() {
            //var oStyle = this.currentStyle? this.currentStyle : window.getComputedStyle(this, null);
            var oStyle = this.currentStyle || window.getComputedStyle(this, null);//IE下获取是2em,Firefox下获取的是24px;浏览器默认1em=16px;
            alert(oStyle.height);
    };
</script>
 </body>
</html>