默认情况
float: left; margin-top: 3px; background: red;
background:blue;
设置了BFC
float: left; margin-top: 3px; background: red;
background:blue; display: flow-root;
创建BFC的方式：
display: flow-root;
display: block; overflow: auto;
column-span: all
column-count: 1
column-width: 20em
contain: layout
position: absolute; width: 30em
display: inline-block; width: 30em
display: table; width: 30em
float: left;
不设置