css transition on hover

CSS
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
.btn {
  background-color: #f4511e;
  border: none;
  color: white;
  padding: 16px 32px;
  text-align: center;
  font-size: 16px;
  margin: 4px 2px;
  opacity: 0.6;
  transition: 0.3s;
}

.btn:hover {opacity: 1}
</style>
</head>
<body>

<h2>Fading Buttons - "Fade in Effect"</h2>

<button class="btn">Hover Over Me</button>

</body>
</html>
/* Simple CSS color transition effect on selector */

div {
	color: white;
  	background-color: black; 
}

div:hover {
	background-color: red;
}


/* Additional transition effects on selector */

div {
	background-color: black;
  	color: white;
  	height: 100px;
  	transition: width 1.5s, height 3s;
  	width: 100px;
  	
}

div:hover {
	background-color: red;
  	height: 300px;	
  	width: 150px;
}
Source

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