BroswerMode/DocumentMode/CompatMode/ 浏览器模式，文档模式，渲染模式

Reference:http://my.oschina.net/junn/blog/125982  

http://blog.csdn.net/jcx5083761/article/details/8284552 

         BorswerMode and DocumentMode are special,which used in IE(the documentmode is added from ie8).the value of the  formar include  5,7,8,9,10 correspond respectively to IE7,8,9,10. the documentmode value
of IE6 is undefined. When you change the borswermode  ,   the documentMode will change accordingly. because the borswermodes have  default documentmode values .in addition, the useragent string sended to server will be changed.Conversely , the change of documentmode
will not change the useragent string or borswermode,it just tell the trident (ie rendering engine) how to render and parse the page code,the page will refresh when you change it.

      "<meta http-equiv="x-ua-compatible" content="IE=7" />"  ,it tells the trident rending the page in documentmode ie7. the
difference between "content="IE=7""  and  IE=EmulateIE7  ,is that the latter is rendering the page
in compatible mode which  takes  account of the doctype statement.

      CompatMode values includes CSS1Compat and BackCompat,which every browser implemented.The CSS1Compat means the CSS rendering
mode is standar mode,the backcompt indicates the doctype is quirks mode.The compatMode will influence the boxing mode .