I made two classes in which one is main and other one is a child div. In main div I give width of 100% background color:red and in child div I give width 100% and background blue and margin top 100px. But in browser red color is not showing up. If I use float in main div then color is appearing in browser but the main class already have width 100% so we are not supposed to give float element in it.
<style>
.main {width:100%; background-color:#FF0000}
.child { width:100%; margin-top:100px; background:#0000FF}
</style>
</head>
<body>
<div class="main">
<div class="child">asfda</div>
</div>
see the fiddle:
Fiddle: http://jsfiddle.net/uT7Vv/
Demo: http://jsfiddle.net/uT7Vv/embedded/result/