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<html xmlns="http://www.w3.org/1999/xhtml">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
*{ margin:0px; padding:0px;}
body{ background-image:url(1.jpg); background-attachment:fixed; background-repeat:no-repeat; background-size:contain;}
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<p id="pp" style="position:fixed;">0</p>
<div style="background-image:url(3.jpg); width:1024px; height:700px; position:relative; top:500px; left:0px;"></div>
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</body>
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<script>
var a =document.getElementById("pp");
window.onscroll=function (){
a.innerHTML=document.documentElement.scrollTop||document.body.scrollTop;
if(parseInt(a.innerHTML)>200)
{
document.body.style.backgroundImage="url(2.jpg)";
}
else
{
document.body.style.backgroundImage="url(1.jpg)";
}
}
</script>原文：http://www.cnblogs.com/wuxiaochao/p/6061550.html