求加黑段怎么理解?!
<style type="text/css">
body{ margin:0; padding:0; font-size:30px}
div{ text-align:center; font-weight:bold}
.main,.footer{ width:20%;margin:0 auto;}
.head{ width:100%; height:100px; background:#ccc}
.main{ height:600px;width:60% ;background:#FCC}
.footer{ height:50px;width:60%; background:#9CF}