clearfix--清楚浮动（来源：HTML5与CSS3基础教程）

/* html5boilerplate.com Clearfix
--------------------------------- */
.clearfix:before,
.clearfix:after {
  content: " ";
  display: table;
}

.clearfix:after {
  clear: both;
}
/*
 * For IE 6/7 only
 * Include this rule to trigger hasLayout and contain floats.
 */
.clearfix {
  *zoom: 1;
}