HTML -

<div id = "cloud"><span class='shadow'></span></div>

CSS -

body {
	background: #ccc;
}
#cloud {
	width: 350px; height: 120px;
	
	background: #f2f9fe;
	background: linear-gradient(top, #f2f9fe 5%, #d6f0fd 100%);
	background: -webkit-linear-gradient(top, #f2f9fe 5%, #d6f0fd 100%);
	background: -moz-linear-gradient(top, #f2f9fe 5%, #d6f0fd 100%);
	background: -ms-linear-gradient(top, #f2f9fe 5%, #d6f0fd 100%);
	background: -o-linear-gradient(top, #f2f9fe 5%, #d6f0fd 100%);
	
	border-radius: 100px;
	-webkit-border-radius: 100px;
	-moz-border-radius: 100px;
	
	position: relative;
	margin: 120px auto 20px;
}

#cloud:after, #cloud:before {
	content: '';
	position: absolute;
	background: #f2f9fe;
	z-index: -1
}

#cloud:after {
	width: 100px; height: 100px;
	top: -50px; left: 50px;
	
	border-radius: 100px;
	-webkit-border-radius: 100px;
	-moz-border-radius: 100px;
}

#cloud:before {
	width: 180px; height: 180px;
	top: -90px; right: 50px;
	
	border-radius: 200px;
	-webkit-border-radius: 200px;
	-moz-border-radius: 200px;
}

.shadow {
	width: 350px;
	position: absolute; bottom: -10px; 
	background: #000;
	z-index: -1;
	
	box-shadow: 0 0 25px 8px rgba(0, 0, 0, 0.4);
	-moz-box-shadow: 0 0 25px 8px rgba(0, 0, 0, 0.4);
	-webkit-box-shadow: 0 0 25px 8px rgba(0, 0, 0, 0.4);
	
	border-radius: 50%;
	-moz-border-radius: 50%;
	-webkit-border-radius: 50%;
}

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Soothing Clouds in CSS3

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