[BFC](https://developer.mozilla.org/zh-CN/docs/Web/Guide/CSS/Block_formatting_context) 1. html根元素 2. float 不为none 3. position: absolute / fixed 4. display: inline-block / table-cell / table-caption / table / table-row / table-row-group / table-header-group / table - foolter-group / flow-root / flex / inline-flex / grid / inline-grid 5. overflow 不为 visible 6. 多列容器 column-count 不为auto 7. column-span 不为1
BFC