一
<div style="width: 400px;background: gray;" id="">
<div style="width: 100px;height: 100px;background: black;" id="">
</div>
</div>
二
<div style="width: 400px;background: gray;" id="">
<div style="float:left;width: 100px;height: 100px;background: black;" id="">
</div>
</div>
三
<div style="float:left;width: 400px;background: gray;" id="">
<div style="float:left;width: 100px;height: 100px;background: black;" id="">
</div>
</div>