IE6対応のマスク関数のjQuery実装を詳しく解説

<p id="main"><a onclick="showBg();" href="#" rel="external nofollow" rel="external nofollow" >点击这里看 jQuery 遮罩层效果.</a></p>
<p id="fullbg"></p>
<p id="dialog">
<p class="close"><a onclick="closeBg();" href="#" rel="external nofollow" rel="external nofollow" >关闭</a></p>
正在加载，请稍后...
</p>

body {
  font-family: Arial, Helvetica, sans-serif;
  font-size: 12px;
  margin: 0;
}
#main {
  height: 1800px;
  padding-top: 90px;
  text-align: center;
}
#fullbg {
  background-color: Gray;
  left: 0px;
  opacity: 0.5;
  position: absolute;
  top: 0px;
  z-index: 3;
  filter: alpha(opacity=50); /* IE6 */
  -moz-opacity: 0.5; /* Mozilla */
  -khtml-opacity: 0.5; /* Safari */
}
#dialog {
  background-color: #FFF;
  border: 1px solid #888;
  display: none;
  height: 200px;
  left: 50%;
  margin: -100px 0 0 -100px;
  padding: 12px;
  position: fixed !important; /* 浮动对话框 */
  position: absolute;
  top: 50%;
  width: 200px;
  z-index: 5;
}
#dialog p {
  margin: 0 0 12px;
}
#dialog p.close {
  text-align: right;
}

<script type="text/javascript" src="http://libs.baidu.com/jquery/2.0.0/jquery.min.js"></script>
<script type="text/javascript">
//显示灰色 jQuery 遮罩层
function showBg() {
  var bh = $("body").height();
  var bw = $("body").width();
  $("#fullbg").css({
    height:bh,
    width:bw,
    display:"block"
  });
  $("#dialog").show();
}
//关闭灰色 jQuery 遮罩
function closeBg() {
  $("#fullbg,#dialog").hide();
}
</script>
<!--[if lte IE 6]>
<script type="text/javascript">
// 浮动对话框
$(document).ready(function() {
  var domThis = $(&#39;#dialog&#39;)[0];
  var wh = $(window).height() / 2;
  $("body").css({
    "background-image": "url(about:blank)",
    "background-attachment": "fixed"
  });
  domThis.style.setExpression(&#39;top&#39;, &#39;eval((document.documentElement).scrollTop + &#39; + wh + &#39;) + "px"&#39;);
});
</script>
<![endif]-->