css hover fade

.btn {
  background-color: #ddd;
  color: black;
  transition: 0.3s;
}

.btn:hover {
  background-color: #3e8e41;
  color: white;
}

/* Changes an element's color on hover */

.selector {
	background-color: black;
}

.selector:hover {
	background-color: blue;
}

.a:hover{background-color: black;}

#cf {
  position:relative;
  height:281px;
  width:450px;
  margin:0 auto;
}

#cf img {
  position:absolute;
  left:0;
  -webkit-transition: opacity 1s ease-in-out;
  -moz-transition: opacity 1s ease-in-out;
  -o-transition: opacity 1s ease-in-out;
  transition: opacity 1s ease-in-out;
}

#cf img.top:hover {
  opacity:0;
}