标签页-导航负margin

/* 标签页菜单 */
div.bs-docs-example {
position: relative;
width: 600px;
margin: 15px auto;
padding: 39px 19px 14px;
background-color: white;
border: 1px solid #DDD;
-webkit-border-radius: 4px;
-moz-border-radius: 4px;
border-radius: 4px;
}

div.bs-docs-example::after {
content: "Example";
position: absolute;
top: -1px;
left: -1px;
padding: 3px 7px;
font-size: 12px;
font-weight: bold;
background-color: #F5F5F5;
border: 1px solid #DDD;
color: #9DA0A4;
-webkit-border-radius: 4px 0 4px 0;
-moz-border-radius: 4px 0 4px 0;
border-radius: 4px 0 4px 0;
}

div.clear {
clear: both;
}

ul.nav-tabs {
border-bottom: 1px solid #DDD;
padding: 0;
margin: 0;
}

ul.nav-tabs li {
margin-bottom: -1px;
float: left;
line-height: 20px;
}

ul.nav-tabs li a {
display: block;
padding: 8px 12px;
line-height: 20px;
font-size: 14px;
color: #08C;
border: 1px solid transparent;
-webkit-border-radius: 4px 4px 0 0;
-moz-border-radius: 4px 4px 0 0;
border-radius: 4px 4px 0 0;
padding-right: 12px;
margin-right: 2px;
}

ul.nav-tabs li a:hover {
background: #eee;
color: #005580;
}

ul.nav-tabs li.active a:hover,ul.nav-tabs li.active a:focus {
color: #555;
cursor: default;
background-color: white;
border: 1px solid #DDD;
border-bottom-color: transparent;
}

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一个完整的margin属性是这么写的margin: top right bottom left;(eg: margin:10px 20px 30px 40px)。在margin属性中一共有两类参考线，top和left的参考线属于一类，right和bottom的参考线属于另一类。top和left是以外元素为参考，right和bottom是以元素本身为参考。margin的位移方向是指margin数值为正值时候的情形，如果是负值则位移方向相反。