Edit in JSFiddle

<div class="clearfix elem">
    <span class="label begin"> &lt;div class="clearfix"&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>
    <section class="elem elem-orange">
         <span class="label begin"> &lt;section&gt;</span>
        <p>当你调整浏览器窗口大小时，布局比以前更酷了！</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>
</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;
}

@media screen and (min-width:600px) {
  nav {
    float: left;
    width: 25%;
  }
  section {
    margin-left: 25%;
  }
}
@media screen and (max-width:599px) {
  nav li {
    display: inline;
  }
}

.clearfix:after {
content: ".";
display: block;
height: 0;
clear: both;
visibility: hidden;
}