.top{ height:100px;background:#9CF}

.head,.main{ width:960px; 【任务1】margin:0 auto;}

.head{ height:100px; background:#F90}

.left{ width:220px; height:600px; background:#ccc;float:left; 【任务2】}

.right{ width:740px; height:600px;background:#FCC; float:right}

.r_sub_left{ width:540px; height:600px; background:#9C3; float:left}

.r_sub_right{ width:200px; height:600px; background:#9FC; 【任务3】float:right;}

.footer{ height:50px; background:#9F9; 【任务4 margin:600px 0 0 0;}