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<div class="container elem">
    <span class="label begin"> &lt;div class="container"&gt; </span>
    <nav class="elem elem-red">
        <span class="label begin"> &lt;nav&gt;</span>
        <ul>
            <li>nav-item1</li>
            <li>nav-item2</li>
            <li>nav-item3</li>
            <li>nav-item4</li>
            <li>nav-item5</li>
            <li>nav-item6</li>       
        </ul>
         <span class="label end"> &lt;/nav&gt; </span>
    </nav>
   <div class="elem elem-red flex-column">
       <span class="label begin"> &lt;div class="flex-column"&gt; </span>
       <section class="elem elem-orange">
         <span class="label begin"> &lt;section&gt;</span>
        <p>Flexbox好容易使用！</p>
         <span class="label end"> &lt;/section&gt; </span>
    </section>
    <section class="elem elem-orange">
         <span class="label begin"> &lt;section&gt;</span>
        <p>一些内容</p>
         <span class="label end"> &lt;/section&gt; </span>
    </section>
       <span class="label end"> &lt;/div&gt; </span>
   </div>
</div>

* {
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}
.elem{
    border:solid #6AC5AC 3px;
    position:relative;
}
.elem-red {
    border: solid #D64078 3px;
}
.elem-orange {
    border: solid #FDC72F 3px;
}
.elem-green {
    border: solid #96C02E 3px;
}
.label{
    position:absolute;
    background-color:#6AC5AC;
    line-height:1em;
    padding:3px;
}
.elem-red > .label{
    background-color:#D64078;
}
.elem-orange > .label{
    background-color:#FDC72F;
}
.begin{
    left:0;
    top:0;
}
.end{
    right:0;
    bottom:0;
}
p{
    margin:2em 0;
    display:block;
}

.container {
  display: -webkit-flex;
  display: flex;
}
nav {
  width: 200px;
}
.flex-column {
  -webkit-flex: 1;
          flex: 1;
}