导航栏组件的差异
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<!-- 导航条 -->
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<div class="navbar-header"> <!--Bootstrap提供的现成的导航栏组件nav -->
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<span>导航栏</span>
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<span bg-default>上面的是用bootstrap2版本写的导航栏，和下方bootstrap4写的导航栏差距有点大</span>
<span bg-default>或者说bootstrap2已经不能在bootstrap4上运行，关于导航栏组件的，要么就要更改组件了</span>
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