知识，可能是我们基础差，老师讲的也是非常基础的一些，但是我们还是没有听懂，仔细研究了一番，反反复复终于

实现了下拉的效果，后来在网上找这方面的教程和代码实例看，发现和自己写的还是有很大的区别，最近一直在深入

JS的学习，还没有学到这块，相信学到这块，自己可以写出更加简洁的下拉菜单的代码。由于JS懂的比较少，大部分

还是用CSS实现的。

继续接着这个系列完成经管系网站的功能构建，这次是给导航栏添加下拉菜单。CSS代码很多，不过为了妥当，

还是粘贴上来了，为了解决看的麻烦的问题，我给每次添加CSS代码的内容都分块了，前面的改动也会用注释标注出

来。

HTML代码：

/*第一次添加网页总体布局的代码*/* { margin:0px; 	padding:0px;}body { padding:0px; 	margin:0px; 	font-size:12px;}#top {	background-image: url(images/head.jpg);	background-repeat: no-repeat;	height: 120px;	width: 998px;	margin-right: auto;	margin-left: auto;}#nav {	background-image: url(images/n_nav_bg.gif);	background-repeat: repeat-x;	height: 37px;	width: 998px;	margin-right: auto;	margin-left: auto;}#adpic {	background-image: url(images/h4.jpg);	background-repeat: no-repeat;	height: 218px;	width: 998px;	margin-right: auto;	margin-left: auto;}#news {	height: 720px;	width: 998px;	margin-right: auto;	margin-left: auto;}#link {	background-image: url(images/n_title_bg.gif);	background-repeat: repeat-x;	height: 40px;	width: 998px;	margin-right: auto;	margin-left: auto;}#copyright {	background-color: #E2E2E2;	height: 80px;	width: 998px;	margin-top: 0px;	margin-right: auto;	margin-left: auto;	border-top-width: 5px;/*设置5像素的上边框*/	border-top-style: solid;	border-top-color: #006042;	margin-bottom: 0px;}#news #news_left {	float: left;	height: 720px;	width: 233px;	margin-right: 5px;}#news #news_mid {	float: left;	height: 720px;	width: 455px;	margin-right: 5px;}#news #news_right {	height: 720px;	width: 300px;	float: left;}#news #news_left #news_left_top {	height: 248px;	width: 233px;}#news #news_left #news_left_mid {	height: 240px;	width: 233px;}#news #news_left #news_left_bottom {	height: 232px;	width: 233px;}#news #news_mid #news_mid_top {	height: 488px;	width: 455px;}#news #news_mid #news_mid_bottom {	height: 232px;	width: 455px;}#news #news_right #news_right_top {	height: 248px;	width: 300px;}#news #news_right #news_right_mid {	height: 240px;	width: 300px;}#news #news_right #news_right_bottom {	height: 232px;	width: 300px;}/*第二次添加导航栏和友情链接栏的代码，下拉菜单我们后面做*/a:link { /*伪类选择页面中全部的超链接标签的未访问的链接状态*/ color:#000000; 	text-decoration:none;}a:visited { /*伪类选择页面中全部的超链接标签的已访问的链接状态*/ color:#000000; 	text-decoration:none;}a:hover{ /*伪类选择页面中全部的超链接标签的鼠标悬停的状态*/	color:#FF0000;	text-decoration:none;	font-weight: bold;	}#nav li {/*后代选择器选择div中行内元素li的设置*/	float: left;	line-height: 37px;	list-style-type:none;	text-align:center;	position:relative;/*给下拉菜单的父元素设置为相对定位*/	}#link #link_left {	font-family: "微软雅黑";	font-size: 14px;	line-height: 32px;	font-weight: bold;	color: #009966;	float: left;	height: 32px;	width: 80px;	padding-left: 10px;}#link #link_right {	line-height: 32px;	float: left;	height: 32px;	width: 908px;}#nav a:link{/*后代选择器选择div中行内选择a设置未访问的链接状态*/	color:#FFFFFF;	text-decoration:none;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	width:px;	height:37px;	display:block;}#nav a:visited{/*后代选择器选择div中行内选择a设置已访问的链接状态*/	color:#FFFFFF;	text-decoration:none;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	width:px;	height:37px;	display:block;}#nav a:hover{/*后代选择器选择div中行内选择a设置鼠标悬浮在链接的的状态*/	color:#FFFFFF;	text-decoration:none;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	background-color: #009966;	width:px;	height:37px;	display:block;}/*第三次添加内容标题栏的代码*/img { /*消除图片的默认格式*/ border:none;}.news_left_titlebar {	background-repeat: no-repeat;	height: 31px;	width: 233px;	border-bottom-width: 1px;	border-bottom-style: dashed;	border-bottom-color: #CCCCCC;	background-color: #FFFFFF;	font-size: 14px;	line-height: 32px;	font-weight: bold;	color: #009966;}.news_left_titlebar img {/*设置图片的格式*/	float: left;	height: 14px;	width: 14px;	margin-top: 9px;	margin-right:5px;}.news_mid_titlebar {	background-image: url(images/n_title_bg.gif);	background-repeat: repeat-x;	height: 32px;	width: 445px;	font-size: 14px;	line-height: 32px;	font-weight: bold;	color: #009966;	padding-left: 10px;}.news_right_titlebar {	background-image: url(images/n_title_bg.gif);	background-repeat: repeat-x;	height: 32px;	width: 290px;	font-size: 14px;	line-height: 32px;	font-weight: bold;	color: #009966;	padding-left: 10px;}#copyright #copyright_top {	height: 20px;	width: 998px;	text-align:center;	line-height:20px;	padding-top:20px;}#copyright #copyright_bottom {	height: 20px;	width: 998px;	text-align:center;	line-height:20px;	padding-bottom:20px;}/*第四次代码左半部分内容的添加*/li { list-style-type:none;}#news #news_left #news_left_top #news_left_top_content {	height: 210px;	width: 233px;	overflow: hidden;	padding-top:3px;	padding-bottom:3px;}#news #news_left #news_left_top #news_left_top_content li {	float: left;	height: 30px;	width: 233px; display:block;}#news #news_left #news_left_top #news_left_top_content li a:link{	color:#999999;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	width:233px;	height:30px;	display:block;	text-align:left;	line-height:30px;	vertical-align:middle;}#news #news_left #news_left_top #news_left_top_content li a:visited{	color:#999999;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	width:233px;	height:30px;	display:block;	text-align:left;	line-height:30px;	vertical-align:middle;}#news #news_left #news_left_top #news_left_top_content li a:hover{	color:#FFFFFF;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	background-color: #009966;	width:233px;	height:30px;	display:block;	text-align:left;	line-height:30px;	vertical-align:middle;}#news #news_left #news_left_mid #news_left_mid_content {	height: 196px;	width: 200px;	overflow: hidden;	padding-top:6px;	padding-bottom:6px;	padding-left:13px;	padding-right:20px;}#news #news_left #news_left_mid #news_left_mid_content a:link{	color:#999999;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	width:100px;	height:27px;	display:block;	text-align:center;	line-height:27px;	vertical-align:middle;}#news #news_left #news_left_mid #news_left_mid_content a:visited{	color:#999999;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	width:100px;	height:27px;	display:block;	text-align:center;	line-height:27px;	vertical-align:middle;}#news #news_left #news_left_mid #news_left_mid_content a:hover{	color:#FFFFFF;	font-family: "微软雅黑";	font-size: 14px;	font-weight: bold;	background-color: #009966;	width:100px;	height:27px;	display:block;	text-align:center;	line-height:27px;	vertical-align:middle;}/*第五次代码中间内容一部分的添加*/#news #news_mid #news_mid_top #news_mid_top_content #news_mid_top_content1 {	background-color:#CCCCCC;	height: 256px;	width: 455px;}#news #news_mid #news_mid_top #news_mid_top_content #news_mid_top_content2 {	height: 232px;	width: 455px;}#news #news_mid #news_mid_top #news_mid_top_content #news_mid_top_content2 li {	line-height: 24px;	border-bottom-width: 1px;	border-bottom-style: dashed;	border-bottom-color: #CCCCCC;}#news #news_mid #news_mid_top #news_mid_top_content #news_mid_top_content2 img {	float: left;	height: 16px;	width: 16px;	margin-top: 5px;}#news #news_mid #news_mid_bottom #news_mid_bottom_content li {	line-height: 24px;	border-bottom-width: 1px;	border-bottom-style: dashed;	border-bottom-color: #CCCCCC;}#news #news_mid #news_mid_bottom #news_mid_bottom_content img {	float: left;	height: 16px;	width: 16px;	margin-top: 5px;}/*第五次代码右半部分内容的添加*/#news #news_right #news_right_top #news_right_top_content li {	line-height: 23px;	border-bottom-width: 1px;	border-bottom-style: dashed;	border-bottom-color: #CCCCCC;}#news #news_right #news_right_mid #news_right_mid_content #news_right_mid_content1 {	height: 100px;	width: 300px;	margin-top: 54px;	margin-bottom: 54px;}#news #news_right #news_right_mid #news_right_mid_content li {	line-height: 24px;	border-bottom-width: 1px;	border-bottom-style: dashed;	border-bottom-color: #CCCCCC;}#news #news_right #news_right_bottom #news_right_bottom_content li {	line-height: 24px;	border-bottom-width: 1px;	border-bottom-style: dashed;	border-bottom-color: #CCCCCC;}/*第六次添加导航栏下拉菜单的代码*/#nav #bzsy {	background-color: #009900;	display: none;	height: 37px;	width: px;	overflow: hidden;	position: absolute;/*下拉列表设置为绝对定位*/	z-index: 99;/*设置元素的堆叠属性*/	left: 0px;	top: 37px;}#nav #bxgk {	background-color: #009900;	display: none;	height: 148px;	width: px;	overflow: hidden;	position: absolute;	z-index: 99;	left: 0px;	top: 37px;}#nav #jyjx {	background-color: #009900;	display: none;	height: 222px;	width: px;	overflow: hidden;	position: absolute;	z-index: 99;	left: 0px;	top: 37px;}#nav #kxyj {	background-color: #009900;	display: none;	height: 111px;	width: px;	overflow: hidden;	position: absolute;	z-index: 99;	left: 0px;	top: 37px;}#nav #xsgz {	background-color: #009900;	display: none;	height: 185px;	width: px;	overflow: hidden;	position: absolute;	z-index: 99;	left: 0px;	top: 37px;}#nav #zsjy {	background-color: #009900;	display: none;	height: 222px;	width: px;	overflow: hidden;	position: absolute;	z-index: 99;	left: 0px;	top: 37px;}#nav #dtjs {	background-color: #009900;	display: none;	height: 111px;	width: px;	overflow: hidden;	position: absolute;	z-index: 99;	left: 0px;	top: 37px;}#nav #kysb {	background-color: #009900;	display: none;	height: 74px;	width: px;	overflow: hidden;	position: absolute;	z-index: 99;	left: 0px;	top: 37px;}#nav #sxjd {	background-color: #009900;	display: none;	height: 185px;	width: px;	overflow: hidden;	position: absolute;	z-index: 99;	left: 0px;	top: 37px;}

function showmenu(str){	var subdiv=document.getElementById(str);	subdiv.style.display="block";}function hidemenu(str){	var subdiv=document.getElementById(str);	subdiv.style.display="none";}