SOURCE

<div class="left">left 200px </div>
<div class="right">right 200px</div>
<div class="center">center</div><!-- 自适应的DIV一定要放在浮动DIV后面 -->

body { margin: 0px; }

.left {
  float: left;
  width: 200px;
  height: 200px;
  background-color: lightblue;
}

.right {
  float: right;
  width: 200px;
  height: 200px;
  background-color: lightblue;
}

.center {
/*   margin-left: 200px;
  margin-right: 200px; */
  height: 200px;
  background-color: pink;
}