往下拖动页面滚动条时，固定住菜单栏(不随其他内容网上滚动，兼容各版本IE、Chrome、Firefox)
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
<script>
// 定义菜单栏离页面顶部的距离，默认为200
var divOffsetTop = 200;
//滚动事件
window.onscroll=function(){
var div = document.getElementById("divId");
// 计算页面滚动了多少（需要区分不同浏览器）
var topVal = 0;
if(window.pageYOffset){//这一条滤去了大部分， 只留了IE678
topVal = window.pageYOffset;
}
else if(document.documentElement.scrollTop ){//IE678 的非quirk模式
topVal = document.documentElement.scrollTop;
}
else if(document.body.scrolltop){//IE678 的quirk模式
topVal = document.body.scrolltop;
}
if(topVal <= divOffsetTop){
div.style.position = "";
}
else {
div.style.position = "fixed";
}
};
// 页面加载完之后，计算菜单栏到页面顶部的实际距离
window.onload=function(){
var div = document.getElementById("divId");
divOffsetTop = div.offsetTop;
};
</script>
</head>
<body>
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<div id="divId" style="width: 500px; height: 40px; line-height: 40px; top: 0px; background: pink">假设这是菜单栏，在页面往下滚动时它会固定住</div>
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</body>
</html>
