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<li><a href="/">首页</a></li>
<li><a href="/category/html-css/">HTML/CSS</a></li>
<li><a href="/category/javascript/">JavaScript</a></li>
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</ul>

$(function() {
var a1 = document.URL;
var a2 = $("#nav a");
for (var i = 0; i < a2.length; i++) {
if (a1.indexOf($(a2[i]).attr("href")) != -1) {
$(a2[i]).parent().addClass("on");
return;
}
}
$(a2[0]).parent().addClass("on");
})

#nav li{
display:inline-block;
float:left;
text-align:center;
height:36px;
padding-left:4px;
line-height:36px;
background:url(images/nav.gif) no-repeat right bottom;
}
#nav li a{
display:block;
color:#777;
padding:0 15px 0 10px;
}
#nav li.on{
font-weight:bold;
background:url(images/nav.gif) no-repeat 0 0;
margin-left:-3px;
}
#nav li.on a{
background:url(images/nav.gif) no-repeat right 0;
}