我想知道如何使我的网站具有带有水平导航栏的固定顶部横幅，因此当人们滚动浏览网站时，横幅和导航保持在其位置，而内容将正常滚动。
HTML：
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>Play - Learn - Grow</title>
<link rel="stylesheet" href="main.css">
</head>
<body class="body">
<span class="banner_h">
<img src="Images\Top_Banner_4.png" alt="Banner" height="150" width ="1240" />
</span>
<nav>
<ul class="nav">
<li><a href="index.html">Home</a></li>
<li><a href="about.html">About Us</a></li>
<li><a href="contact.html">Contact Us</a></li>
<li><a href="membership.html">Become a Member</a></li>
<li><a href="borrow.html">Borrow Toys</a></li>
<li><a href="policies.html">Our Policies</a></li>
<li><a href="sitemap.html">Site Map</a></li>
</ul>
</nav>
<span class="banner_l">
<img src="Images\Side_Banner.jpg" alt="Banner" />
</span>
<span class="banner_r">
<img src="Images\Side_Banner.jpg" alt="Banner" />
</span>
<h2 class="headers">Welcome to the Home Page!</h2>
<div class="container">
Our aim is to provide the children of the community with an ever-changing variety of educational and fun toys to enhance
their cognitive, social, emotional and physical development in the important first six years of their lives.
<br><br><span class="Links">Be sure to check out our Wikispace site with more information <a href="http://mysocialmediatools-pn.wikispaces.com/">here</a>!</span>
</div>
<div id="content"></div>
<div id="footer">
Copyright © 2013
</div>
</body>
</html>
CSS：
/* Entire Document CSS */
html{
height: 100%;
}
/* Header CSS */
.headers{
color: #FFD89A;
text-align: center;
padding: 10px;
}
/* Body CSS */
.body{
background-color: #61B329;
height: 50%;
color: #FFFFFF;
}
.container{
margin: 0 auto 0 auto;
width: 50em;
text-align: center;
padding-bottom: 500px;
height: 50%;
}
/* Navigation CSS */
.nav {
display: inline-block;
background-color: #00B2EE;
border: 1px solid #000000;
border-width: 1px 0px;
margin: 0;
padding: 0;
min-width: 1000px;
width: 100%;
}
.nav li {
list-style-type: none;
width: 14.28%;
float: left;
}
.nav a {
display: inline-block;
padding: 10px 0;
width: 100%;
text-align: center;
}
/* Banner / Picture CSS / Text in Images */
.banner_l{
float: left;
}
.banner_r{
float: right;
}
.banner_h{
display: block;
width: 100%;
}
.banner_h img{
width: 100%;
}
/* Footer CSS */
#footer {
clear: both;
position: relative;
z-index: 10;
height: 3em;
margin-top: -3em;
}
#content {
padding-bottom: 3em;
}
/* Link CSS */
a:link{
color: #FFFFFF;
text-decoration: none;
}
a:visited{
color: #FFFFFF;
text-decoration: none;
}
a:hover{
background-color: #028482;
color: #FFFFFF;
text-decoration: underline;
}
a:active{
background-color: #FCDC3B;
color: #AA00FF;
text-decoration: overline;
}
.Links A:hover{
color: #028482;
background-color: transparent;
text-decoration: underline overline;
}
谢谢！