三列浮动布局为啥要嵌套两层?
摘自《精通css》8.3.2节——
<div class="content">
<div class="primary">
<div class="primary">
<-- your primary primary content goes here -->
</div>
<div class="secondary">
<-- your secondary primary content goes here -->
</div>
</div>
<div class="secondary”>
<!--navigation and secondary content goes here -->
</div>
</div>
<div class="content">
<div class="left_one"></div>
<div class="left_two"></div>
<div class="right"></div>
</div>