#top{width:100%;
overflow:hidden;
border-bottom:1px solid #dadad9;
background:#fff;
float:left;
【任务2】position：fixed;margin-top:0;margin-left:0;
text-align:center;}
#top input{border:none;}
.left{float:left; margin-left:10px;}
.search{float:left; margin-left:10px; margin-top:7px;}
.select_btn{background:url(images/select_top.gif) no-repeat; width:57px; height:42px; cursor:pointer; float:left;}
.search_input{background:url(images/input_search.gif) no-repeat; width:104px; height:28px; float:left; padding-left:5px;}
.search_btn{background:url(images/search_btn.gif) no-repeat; width:29px; height:28px; float:left;}
.right{float:right; margin-right:10px; padding:5px 10px 0; border-left:1px solid #f5f5f5; border-right:1px solid #f5f5f5; height:37px;}
.topbtn{background:url(images/top_btn.gif) no-repeat; width:54px; height:30px; line-height:30px; color:#444;}
.margin_l_10{margin-left:10px;}
.margin_t_5{margin-top:5px;}
.center{margin:0 15px;}
.center_r{margin-right:15px;}
#mainbody{width:80%;
overflow:hidden;
【任务2】margin:0 auto;
【任务3】padding:63px;}
.content{width:236px; border:1px solid #dcdcda; float:left;}
.content li{margin-bottom:15px; text-align:center;}
.content li h2{text-align:left; padding:12px 15px; font-weight:normal; color:#444; background:#fff; margin-top:-3px; background:#fafafa;}
.content li dl{border-top:1px solid #f2f2f2; text-align:left;}
.content li dt{padding:15px; float:left; border-right:1px solid #f2f2f2;}
.content li dd{padding-top:18px; text-indent:15px;}
.content li dd a{color:#9e7e6b;}
#footer{width:100%; background:#fff; text-align:center; padding:20px 0; color:#999; border-top:#ccc; margin-top:20px;}
</style>
</head>
<body>
<div id="top">
<div class="left">
<div class="select_btn"></div>
<div class="search">
<input type="text" class="search_input" />
<input type="button" class="search_btn" />
</div>
</div>
<div class="right">
<input type="button" value="登录" class="topbtn" />
<input type="button" value="注册" class="topbtn margin_l_10" />
</div>
</div>
<div id="mainbody">
<div class="content">
当我任务2中写 margin-left 时，任务3 写margin时无法实现 mainbody 下移，只能通过padding:63px.
而写left、top 时，任务3就可以用margin下移。
请问这两种情况怎么理解呢