Edit in JSFiddle

 .animate_opacity{
  	transition-property:opacity;
	transition-duration:0.4s;

  }

  .animate_opacity:hover{
  	opacity:0.4;
  }

/*** following is just box styling ***/
.box {
	height: 50px;
	width: 200px;	
	padding: 5px;
	border-radius:10px;
	margin: 10px;
	cursor:pointer;
	border:2px solid #0079FF;
	letter-spacing: -0.5;
	font: 20px/50px 'PT Sans', sans-serif;
	text-align: center;
    background-color: #0079FF;
    color:#fff;
    -webkit-box-shadow: 1px 1px 2px #aaa, 1px 1px 1px #6cf inset;
		box-shadow: 1px 1px 2px #aaa, 1px 1px 1px #6cf inset;
  }
 

<div class="box animate_opacity">
  Transparency Demo
  </div>