什么是 Css Hack？ie6,7,8 的 hack 分别是什么？

#test{
    width:300px;
    height:300px;
    background-color:blue;      /_firefox_/
    background-color:red\9;      /_all ie_/
    background-color:yellow;    /_ie8_/
    +background-color:pink;        /_ie7_/
    \_background-color:orange;       /_ie6_/   
}

 :root #test { background-color:purple\9; }  /*ie9*/

@media all and (min-width:0px)

     { #test {background-color:black;} }  /*opera*/

@media screen and (-webkit-min-device-pixel-ratio:0)

{ #test {background-color:gray;} }       /*chrome and safari*/