※★CSS
.box81,
.box82 {
width: 100px;
background-color:#fcccff;
float: left;
}
.box811,
.box822 {
width: 100px;
min-height: 100px;/**/
background-color:#fcccff;
float: left;
}
※★HTML
<div class="box81">BOX81</div>
<div class="box82">BOX82</div>
<div class="box83">BOX83</div>
<div class="box811">BOX81</div>
<div class="box822">BOX82</div>
<div class="box833">BOX83</div>