======================上下两行四个区域布局=====================
其中:
div{
border:0px;
margin:0px;
}
<div style="clear:both;width:100%;">
<div style="width:50%;float:left;">left1</div>
<div style="width:50%;float:right;">right1</div>
</div>
<div style="clear:both;width:100%;">
<div style="width:50%;float:left;">left2</div>
<div style="width:50%;float:right;">right2</div>
</div>