0805 self-summary
A. Absolute positioning
position: absolute;
/*绝对定位：
1、定位属性值：absolute
2、在页面中不再占位(浮起来了)，就无法继承父级的宽度(必须自己自定义宽度)
3、一旦定位后，定位的布局方位 top、bottom、left、right都能参与布局
4、绝对定位的参考系是最近的定位父级(不是父级中的哪一点，而是四边参照四边)
5、左右同时存在，取左；同理上下取上
6、当父级定位了，子级参照父级定位，又可以重新获取父级宽度(也可以在计算中拿到父级高度)
*/position:fixed
Generate absolute positioning of elements relative to the browser window positioning.
II. Relative positioning
position: relative;
Parent (parent recent) positioned opposite the object
1) does not affect the layout itself
2) Auxiliary absolute positioning themselves Layout
Three default positioning
position: static
Defaults. No positioning element occurs in the normal flow (ignoring top, bottom, left, right or z-index statement).
IV. Inherits targeting
position:inherit
Provision should inherit the value of the position property from the parent element.
V. scrollbar
overflow
- hidden: no scroll bars, and only the content specified content area.
visible: The default value is no scroll bar, how about this property defines overflow element's content area will be processed and displayed.
scroll: this attribute defines overflow element's content area will be how to deal with. If the value scroll, regardless of whether the user agent will provide a scrolling mechanism. Therefore, it is possible even if you can put aside all the content elements box will appear scroll bar.
auto: scroll bars, and only the content specified content area.