依据分辨率区分手机、平板、pc

/*手机*/
@media screen and (max-width:600px){
   #header,#content,#footer{width:400px;}
   .right,.center{margin-top:10px;}
   .left,.right{height:100px;}
   .center{height:200px;}
}
/*平板*/
@media screen and (min-width:600px) and (max-width:960px){
    #header,#content,#footer{width:600px;}
.right{display:none;}
.left,.center{height:400px;float:left;}
.left{width:160px;margin-right:10px;}
.center{width:430px;}
}
/*PC*/
@media screen and (min-width:960px){
    #header,#content,#footer{width:960px;}
.left,.center,.right{height:400px;float:left;}
.left{width:200px;margin-right:10px;}
.center{width:540px;margin-right:10px;}
.right{width:200px;}
}