![這裏寫圖片描述]()
<script src="jquery-1.12.2.js"></script>
<script>
$(function () {
$(window).scroll(function () {
if($(document).scrollTop()>$(".top").height()){
$(".nav").css({"position":"fixed","top":0});
$(".main").css("marginTop",$(".nav").height());
}else{
$(".nav").css("position","static");
$(".main").css("marginTop",0);
}
});
});
</script>
</head>
<body>
<div class="top">
<img src="images/top.png" />
</div>
<div class="nav">
<img src="images/nav.png" />
</div>
<div class="main">
<img src="images/main.png" />
</div>
