1.hover

移入其父元素.navI时，触发鼠标的hover态，给父元素添加样式为position:relative;z-index:1;。从而提升了层级z-index。在其子元素导航内容的层级比拼中，“子凭父贵”，父元素层级高的，其导航内容在重叠状态中显示在最上面

<style>
body,p{margin: 0;}
h2{margin: 0;font-size:100%;}
ul{margin: 0;padding: 0;list-style: none;} 
a{text-decoration: none;color:inherit;}
.box{width: 572px;border: 1px solid #999;overflow: hidden;}
.nav{margin-left: -1px;font: 14px "微软雅黑";overflow: hidden;background-color: #f1f1f1;}
.navI{float: left;width: 33.333%;box-sizing: border-box;}
.navI-tit{line-height: 40px;text-align: center;cursor: pointer;border-left: 1px solid #cecece;border-bottom: 1px solid #cecece;}
.navI-txt{width: 572px;height:200px;text-indent:2em;line-height: 2;background:#fff;}
.ml1{margin-left: -100%;}
.ml2{margin-left: -200%;}
.navI_active{position:relative;z-index:1;}
/*重点代码*/
.navI:hover{position:relative;z-index:1;}
.navI:hover .navI-tit{background:#fff;border-bottom:none;}
</style>

<p class="box">
 <ul class="nav">
 <li class="navI navI_active">
 <h2 class="navI-tit">课程</h2>
 <p class="navI-txt">课程内容</p>
 </li>
 <li class="navI">
 <h2 class="navI-tit">学习计划</h2>
 <p class="navI-txt ml1">学习计划内容</p>
 </li>
 <li class="navI">
 <h2 class="navI-tit">技能图谱</h2>
 <p class="navI-txt ml2">技能图谱内容</p>
 </li>
 </ul> 
</p>

2.锚点

点击导航标题时，触发target伪类，改变对应的导航内容的层级z-index，从而使当前导航内容在三个导航内容中胜出，在最上层显示；与此同时，改变当前导航标题的样式

<style>
body,p{margin: 0;}
h2{margin: 0;font-size:100%;}
ul{margin: 0;padding: 0;list-style: none;} 
a{text-decoration: none;color:inherit;}
.box{width: 572px;border: 1px solid #999;overflow: hidden;}
.nav{margin-left: -1px;font: 14px "微软雅黑";overflow: hidden;background-color: #f1f1f1;}
.navI{float: left;width: 33.333%;box-sizing: border-box;position:relative;}
.navI-tit{position:absolute;top:0;left:0;right:0;box-sizing: border-box;line-height: 40px;height: 40px;text-align: center;cursor: pointer;border-left: 1px solid #cecece;border-bottom: 1px solid #cecece;}
.navI-txt{width: 572px;height:200px;margin-top: 40px;text-indent:2em;line-height: 2;background:#fff;}
.ml1{margin-left: -100%;}
.ml2{margin-left: -200%;}
.navI_active{z-index:1;}
/*重点代码*/
.navI-txt:target{position:relative;z-index:1;}
.navI-txt:target ~ .navI-tit{background:#fff;border-bottom:none;}
</style>
<p class="box">
 <ul class="nav">
 <li class="navI navI_active">
 <p class="navI-txt" id="kc">课程内容</p>
 <a class="navI-tit" href="#kc">课程</a>
 </li>
 <li class="navI">
 <p class="navI-txt ml1" id="xx">学习计划内容</p>
 <a class="navI-tit" href="#xx">学习计划</a>
 </li>
 <li class="navI">
 <p class="navI-txt ml2" id="jn">技能图谱内容</p>
 <a class="navI-tit" href="#jn">技能图谱</a>
 </li>
 </ul> 
</p>

3.radio

点击导航标题时，触发checked伪类，改变对应的导航内容的层级z-index，从而使当前导航内容在三个导航内容中胜出，在最上层显示；与此同时，改变当前导航标题的样式

<style>
body,p{margin: 0;}
ul{margin: 0;padding: 0;list-style: none;}
a{text-decoration: none;color: inherit;}
input{margin: 0;padding: 0;border:none;}
.box{width:572px;border:1px solid #999;font:14px "微软雅黑";overflow:hidden;}
.nav-tit{margin-left: -1px;height: 40px;line-height: 40px;text-align: center;background-color: #f1f1f1;overflow: hidden;}
.nav-titI{box-sizing: border-box;float: left;width: 33.333%;border-left: 1px solid #cecece;border-bottom: 1px solid #cecece;cursor: pointer;}
.nav-txt{height: 200px;}
.nav-txtI{height: 200px;display:block;width: 100%;text-indent: 2em; line-height: 2;}
/*重点内容*/
.nav-txt{overflow: hidden;}
.nav-titI:hover{background-color: #fff;border-bottom:none;}
</style>
<p class="box">
 <nav class="nav-tit">
 <label class="nav-titI" for="kc">课程</label>
 <label class="nav-titI" for="xx">学习计划</label>
 <label class="nav-titI" for="jn">技能图谱</label>
 </nav>
 <nav class="nav-txt">
 <input class="nav-txtI nav-txtI_active" id="kc" value="课程内容" readonly>
 <input class="nav-txtI" id="xx" value="学习计划内容" readonly>
 <input class="nav-txtI" id="jn" value="技能图谱内容" readonly>
 </nav>
</p>