<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/htmlcharset=utf-8" />
<title>二级dropdown弹出菜单--A CROSS BROWSER DROP DOWN CASCADING VALIDATING MENU</title>
<style type="text/css">
/* common styling */
/* set up the overall width of the menu div, the font and the margins */
.menu {
font-family: arial, sans-serif
width:750px
margin:0
margin:50px 0
}
/* remove the bullets and set the margin and padding to zero for the unordered list */
.menu ul {
padding:0
margin:0
list-style-type: none
}
/* float the list so that the items are in a line and their position relative so that the drop down list will appear in the right place underneath each list item */
.menu ul li {
float:left
position:relative
}
/* style the links to be 104px wide by 30px high with a top and right border 1px solid white. Set the background color and the font size. */
.menu ul li a, .menu ul li a:visited {
display:block
text-align:center
text-decoration:none
width:104px
height:30px
color:#000
border:1px solid #fff
border-width:1px 1px 0 0
background:#c9c9a7
line-height:30px
font-size:11px
}
/* make the dropdown ul invisible */
.menu ul li ul {
display: none
}
/* specific to non IE browsers */
/* set the background and foreground color of the main menu li on hover */
.menu ul li:hover a {
color:#fff
background:#b3ab79
}
/* make the sub menu ul visible and position it beneath the main menu list item */
.menu ul li:hover ul {
display:block
position:absolute
top:31px
left:0
width:105px
}
/* style the background and foreground color of the submenu links */
.menu ul li:hover ul li a {
display:block
background:#faeec7
color:#000
}
/* style the background and forground colors of the links on hover */
.menu ul li:hover ul li a:hover {
background:#dfc184
color:#000
}
</style>
<!--[if lte IE 6]>
<style type="text/css">
/* styling specific to Internet Explorer IE5.5 and IE6. Yet to see if IE7 handles li:hover */
/* Get rid of any default table style */
table {
border-collapse:collapse
margin:0
padding:0
}
/* ignore the link used by 'other browsers' */
.menu ul li a.hide, .menu ul li a:visited.hide {
display:none
}
/* set the background and foreground color of the main menu link on hover */
.menu ul li a:hover {
color:#fff
background:#b3ab79
}
/* make the sub menu ul visible and position it beneath the main menu list item */
.menu ul li a:hover ul {
display:block
position:absolute
top:32px
left:0
width:105px
}
/* style the background and foreground color of the submenu links */
.menu ul li a:hover ul li a {
background:#faeec7
color:#000
}
/* style the background and forground colors of the links on hover */
.menu ul li a:hover ul li a:hover {
background:#dfc184
color:#000
}
</style>
<![endif]-->
</head>
<body>
<div class="menu">
<ul>
<li><a class="hide" href="../menu/index.html">DEMOS</a>
<!--[if lte IE 6]>
<a href="../menu/index.html">DEMOS
<table><tr><td>
<![endif]-->
<ul>
<li><a href="../menu/zero_dollars.html" title="The zero dollar ads page">zero dollars</a></li>
<li><a href="../menu/embed.html" title="Wrapping text around images">wrapping text</a></li>
<li><a href="../menu/form.html" title="Styling forms">styled form</a></li>
<li><a href="../menu/nodots.html" title="Removing active/focus borders">active focus</a></li>
<li><a href="../menu/shadow_boxing.html" title="Multi-position drop shadow">shadow boxing</a></li>
<li><a href="../menu/old_master.html" title="Image Map for detailed information">image map</a></li>
<li><a href="../menu/bodies.html" title="fun with background images">fun backgrounds</a></li>
<li><a href="../menu/fade_scroll.html" title="fade-out scrolling">fade scrolling</a></li>
<li><a href="../menu/em_images.html" title="em size images compared">em sized images</a></li>
</ul>
<!--[if lte IE 6]>
</td></tr></table>
</a>
<![endif]-->
</li>
<li><a class="hide" href="index.html">MENUS</a>
<!--[if lte IE 6]>
<a href="index.html">MENUS
<table><tr><td>
<![endif]-->
<ul>
<li><a href="spies.html" title="a coded list of spies">spies menu</a></li>
<li><a href="vertical.html" title="a horizontal vertical menu">vertical menu</a></li>
<li><a href="expand.html" title="an enlarging unordered list">enlarging list</a></li>
<li><a href="enlarge.html" title="an unordered list with link images">link images</a></li>
<li><a href="cross.html" title="non-rectangular links">non-rectangular</a></li>
<li><a href="jigsaw.html" title="jigsaw links">jigsaw links</a></li>
<li><a href="circles.html" title="circular links">circular links</a></li>
</ul>
<!--[if lte IE 6]>
</td></tr></table>
</a>
<![endif]-->
</li>
<li><a class="hide" href="../layouts/index.html">LAYOUTS</a>
<!--[if lte IE 6]>
<a href="../layouts/index.html">LAYOUTS
<table><tr><td>
<![endif]-->
<ul>
<li><a href="../layouts/bodyfix.html" title="Cross browser fixed layout">Fixed 1</a></li>
<li><a href="../layouts/body2.html" title="Cross browser fixed layout">Fixed 2</a></li>
<li><a href="../layouts/body4.html" title="Cross browser fixed layout">Fixed 3</a></li>
<li><a href="../layouts/body5.html" title="Cross browser fixed layout">Fixed 4</a></li>
<li><a href="../layouts/minimum.html" title="A simple minimum width layout">minimum width</a></li>
</ul>
<!--[if lte IE 6]>
</td></tr></table>
</a>
<![endif]-->
</li>
<li><a class="hide" href="../boxes/index.html">BOXES</a>
<!--[if lte IE 6]>
<a href="../boxes/index.html">BOXES
<table><tr><td>
<![endif]-->
<ul>
<li><a href="spies.html" title="a coded list of spies">spies menu</a></li>
<li><a href="vertical.html" title="a horizontal vertical menu">vertical menu</a></li>
<li><a href="expand.html" title="an enlarging unordered list">enlarging list</a></li>
<li><a href="enlarge.html" title="an unordered list with link images">link images</a></li>
<li><a href="cross.html" title="non-rectangular links">non-rectangular</a></li>
<li><a href="jigsaw.html" title="jigsaw links">jigsaw links</a></li>
<li><a href="circles.html" title="circular links">circular links</a></li>
</ul>
<!--[if lte IE 6]>
</td></tr></table>
</a>
<![endif]-->
</li>
<li><a class="hide" href="../mozilla/index.html">MOZILLA</a>
<!--[if lte IE 6]>
<a href="../mozilla/index.html">MOZILLA
<table><tr><td>
<![endif]-->
<ul>
<li><a href="../mozilla/dropdown.html" title="A drop down menu">drop down menu</a></li>
<li><a href="../mozilla/cascade.html" title="A cascading menu">cascading menu</a></li>
<li><a href="../mozilla/content.html" title="Using content:">content:</a></li>
<li><a href="../mozilla/moxbox.html" title=":hover applied to a div">mozzie box</a></li>
<li><a href="../mozilla/rainbow.html" title="I can build a rainbow">rainbow box</a></li>
<li><a href="../mozilla/snooker.html" title="Snooker cue">snooker cue</a></li>
<li><a href="../mozilla/target.html" title="Target Practise">target practise</a></li>
<li><a href="../mozilla/splittext.html" title="Two tone headings">two tone headings</a></li>
<li><a href="../mozilla/shadow_text.html" title="Shadow text">shadow text</a></li>
</ul>
<!--[if lte IE 6]>
</td></tr></table>
</a>
<![endif]-->
</li>
<li><a class="hide" href="../ie/index.html">EXPLORER</a>
<!--[if lte IE 6]>
<a href="../ie/index.html">EXPLORER
<table><tr><td>
<![endif]-->
<ul>
<li><a href="../ie/exampleone.html" title="Example one">example one</a></li>
<li><a href="../ie/weft.html" title="Weft fonts">weft fonts</a></li>
<li><a href="../ie/exampletwo.html" title="Vertical align">vertical align</a></li>
</ul>
<!--[if lte IE 6]>
</td></tr></table>
</a>
<![endif]-->
</li>
<li><a class="hide" href="../opacity/index.html">OPACITY</a>
<!--[if lte IE 6]>
<a href="../opacity/index.html">OPACITY
<table><tr><td>
<![endif]-->
<ul>
<li><a href="../opacity/colours.html" title="colour wheel">opaque colours</a></li>
<li><a href="../opacity/picturemenu.html" title="a menu using opacity">opaque menu</a></li>
<li><a href="../opacity/png.html" title="partial opacity">partial opacity</a></li>
<li><a href="../opacity/png2.html" title="partial opacity II">partial opacity II</a></li>
</ul>
<!--[if lte IE 6]>
</td></tr></table>
</a>
<![endif]-->
</li>
</ul>
<!-- clear the floats if required -->
<div class="clear"></div>
</div>
</body>
</html>
3种实现方式：
点击导航，页面刷新，加载完成后重新使导航内容为刷新前的内容，通过url中传参就可以达到了。
采用单页面的方式，内容都写在页面中，初始化时，仅显示当前内容，其他内容隐藏，通过点击导航条来切换需要显示的内容。
采用iframe的方式，要显示的内容在主html中，内容显示在html的iframe标签中，通过改变iframe的src来加载不同的内容。
目前很多网站都是采用第一种方式；论体验，第二种方式好；论方便，选择1,3种方式！
举个例子吧，任意一本教科书里面，每一章算是一级导航菜单，那每一章里面的每一节就是二级导航菜单，每一节里面还可以有一段算是三级导航菜单，以此类推可以有N级导航菜单。每级菜单里面可以有div和table,dl,dt,dd等等。
